| Message ID | 20240410142948.2817554-3-yi.zhang@huaweicloud.com |
|---|---|
| State | New |
| Headers | show |
| Series | ext4: use iomap for regular file's buffered IO path and enable large folio | expand |
Zhang Yi <yi.zhang@huaweicloud.com> writes: > From: Zhang Yi <yi.zhang@huawei.com> > > Now we lookup extent status entry without holding the i_data_sem before > inserting delalloc block, it works fine in buffered write path and > because it holds i_rwsem and folio lock, and the mmap path holds folio > lock, so the found extent locklessly couldn't be modified concurrently. > But it could be raced by fallocate since it allocate block whitout > holding i_rwsem and folio lock. > > ext4_page_mkwrite() ext4_fallocate() > block_page_mkwrite() > ext4_da_map_blocks() > //find hole in extent status tree > ext4_alloc_file_blocks() > ext4_map_blocks() > //allocate block and unwritten extent > ext4_insert_delayed_block() > ext4_da_reserve_space() > //reserve one more block > ext4_es_insert_delayed_block() > //drop unwritten extent and add delayed extent by mistake > > Then, the delalloc extent is wrong until writeback, the one more > reserved block can't be release any more and trigger below warning: > > EXT4-fs (pmem2): Inode 13 (00000000bbbd4d23): i_reserved_data_blocks(1) not cleared! > > Hold i_data_sem in write mode directly can fix the problem, but it's > expansive, we should keep the lockless check and check the extent again > once we need to add an new delalloc block. Hi Zhang, It's a nice finding. I was wondering if this was caught in any of the xfstests? I have reworded some of the commit message, feel free to use it if you think this version is better. The use of which path uses which locks was a bit confusing in the original commit message. <reworded from your original commit msg> ext4_da_map_blocks(), first looks up the extent status tree for any extent entry with i_data_sem held in read mode. It then unlocks i_data_sem, if it can't find an entry and take this lock in write mode for inserting a new da entry. This is ok between - 1. ext4 buffered-write path v/s ext4_page_mkwrite(), because of the folio lock 2. ext4 buffered write path v/s ext4 fallocate because of the inode lock. But this can race between ext4_page_mkwrite() & ext4 fallocate path - ext4_page_mkwrite() ext4_fallocate() block_page_mkwrite() ext4_da_map_blocks() //find hole in extent status tree ext4_alloc_file_blocks() ext4_map_blocks() //allocate block and unwritten extent ext4_insert_delayed_block() ext4_da_reserve_space() //reserve one more block ext4_es_insert_delayed_block() //drop unwritten extent and add delayed extent by mistake Then, the delalloc extent is wrong until writeback and the extra reserved block can't be released any more and it triggers below warning: EXT4-fs (pmem2): Inode 13 (00000000bbbd4d23): i_reserved_data_blocks(1) not cleared! This patch fixes the problem by looking up extent status tree again while the i_data_sem is held in write mode. If it still can't find any entry, then we insert a new da entry into the extent status tree. > > Cc: stable@vger.kernel.org > Signed-off-by: Zhang Yi <yi.zhang@huawei.com> > --- > fs/ext4/inode.c | 19 +++++++++++++++++++ > 1 file changed, 19 insertions(+) > > diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c > index 6a41172c06e1..118b0497a954 100644 > --- a/fs/ext4/inode.c > +++ b/fs/ext4/inode.c > @@ -1737,6 +1737,7 @@ static int ext4_da_map_blocks(struct inode *inode, sector_t iblock, > if (ext4_es_is_hole(&es)) > goto add_delayed; > > +found: > /* > * Delayed extent could be allocated by fallocate. > * So we need to check it. > @@ -1781,6 +1782,24 @@ static int ext4_da_map_blocks(struct inode *inode, sector_t iblock, > > add_delayed: > down_write(&EXT4_I(inode)->i_data_sem); > + /* > + * Lookup extents tree again under i_data_sem, make sure this > + * inserting delalloc range haven't been delayed or allocated > + * whitout holding i_rwsem and folio lock. > + */ page fault path (ext4_page_mkwrite does not take i_rwsem) and fallocate path (no folio lock) can race. Make sure we lookup the extent status tree here again while i_data_sem is held in write mode, before inserting a new da entry in the extent status tree. > + if (ext4_es_lookup_extent(inode, iblock, NULL, &es)) { > + if (!ext4_es_is_hole(&es)) { > + up_write(&EXT4_I(inode)->i_data_sem); > + goto found; > + } > + } else if (!ext4_has_inline_data(inode)) { > + retval = ext4_map_query_blocks(NULL, inode, map); > + if (retval) { > + up_write(&EXT4_I(inode)->i_data_sem); > + return retval; > + } > + } > + > retval = ext4_insert_delayed_block(inode, map->m_lblk); > up_write(&EXT4_I(inode)->i_data_sem); > if (retval) > -- > 2.39.2
Ritesh Harjani (IBM) <ritesh.list@gmail.com> writes: > Zhang Yi <yi.zhang@huaweicloud.com> writes: > >> From: Zhang Yi <yi.zhang@huawei.com> >> >> Now we lookup extent status entry without holding the i_data_sem before >> inserting delalloc block, it works fine in buffered write path and >> because it holds i_rwsem and folio lock, and the mmap path holds folio >> lock, so the found extent locklessly couldn't be modified concurrently. >> But it could be raced by fallocate since it allocate block whitout >> holding i_rwsem and folio lock. >> >> ext4_page_mkwrite() ext4_fallocate() >> block_page_mkwrite() >> ext4_da_map_blocks() >> //find hole in extent status tree >> ext4_alloc_file_blocks() >> ext4_map_blocks() >> //allocate block and unwritten extent >> ext4_insert_delayed_block() >> ext4_da_reserve_space() >> //reserve one more block >> ext4_es_insert_delayed_block() >> //drop unwritten extent and add delayed extent by mistake >> >> Then, the delalloc extent is wrong until writeback, the one more >> reserved block can't be release any more and trigger below warning: >> >> EXT4-fs (pmem2): Inode 13 (00000000bbbd4d23): i_reserved_data_blocks(1) not cleared! >> >> Hold i_data_sem in write mode directly can fix the problem, but it's >> expansive, we should keep the lockless check and check the extent again >> once we need to add an new delalloc block. > > Hi Zhang, > > It's a nice finding. I was wondering if this was caught in any of the > xfstests? > > I have reworded some of the commit message, feel free to use it if you > think this version is better. The use of which path uses which locks was > a bit confusing in the original commit message. > > <reworded from your original commit msg> > > ext4_da_map_blocks(), first looks up the extent status tree for any > extent entry with i_data_sem held in read mode. It then unlocks > i_data_sem, if it can't find an entry and take this lock in write > mode for inserting a new da entry. Sorry about this above paragraph. I messed this paragraph. Here is the correct version of this. ext4_da_map_blocks looks up for any extent entry in the extent status tree (w/o i_data_sem) and then the looks up for any ondisk extent mapping (with i_data_sem in read mode). If it finds a hole in the extent status tree or if it couldn't find any entry at all, it then takes the i_data_sem in write mode to add a da entry into the extent status tree. This can actually race with page mkwrite & fallocate path. Note that this is ok between... <and the rest can remain same> > > This is ok between - > 1. ext4 buffered-write path v/s ext4_page_mkwrite(), because of the > folio lock > 2. ext4 buffered write path v/s ext4 fallocate because of the inode > lock. > > But this can race between ext4_page_mkwrite() & ext4 fallocate path - > > ext4_page_mkwrite() ext4_fallocate() > block_page_mkwrite() > ext4_da_map_blocks() > //find hole in extent status tree > ext4_alloc_file_blocks() > ext4_map_blocks() > //allocate block and unwritten extent > ext4_insert_delayed_block() > ext4_da_reserve_space() > //reserve one more block > ext4_es_insert_delayed_block() > //drop unwritten extent and add delayed extent by mistake > > Then, the delalloc extent is wrong until writeback and the extra > reserved block can't be released any more and it triggers below warning: > > EXT4-fs (pmem2): Inode 13 (00000000bbbd4d23): i_reserved_data_blocks(1) not cleared! > > This patch fixes the problem by looking up extent status tree again > while the i_data_sem is held in write mode. If it still can't find > any entry, then we insert a new da entry into the extent status tree. > >> >> Cc: stable@vger.kernel.org >> Signed-off-by: Zhang Yi <yi.zhang@huawei.com> >> --- >> fs/ext4/inode.c | 19 +++++++++++++++++++ >> 1 file changed, 19 insertions(+) >> >> diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c >> index 6a41172c06e1..118b0497a954 100644 >> --- a/fs/ext4/inode.c >> +++ b/fs/ext4/inode.c >> @@ -1737,6 +1737,7 @@ static int ext4_da_map_blocks(struct inode *inode, sector_t iblock, >> if (ext4_es_is_hole(&es)) >> goto add_delayed; >> >> +found: >> /* >> * Delayed extent could be allocated by fallocate. >> * So we need to check it. >> @@ -1781,6 +1782,24 @@ static int ext4_da_map_blocks(struct inode *inode, sector_t iblock, >> >> add_delayed: >> down_write(&EXT4_I(inode)->i_data_sem); >> + /* >> + * Lookup extents tree again under i_data_sem, make sure this >> + * inserting delalloc range haven't been delayed or allocated >> + * whitout holding i_rwsem and folio lock. >> + */ > > page fault path (ext4_page_mkwrite does not take i_rwsem) and fallocate > path (no folio lock) can race. Make sure we lookup the extent status > tree here again while i_data_sem is held in write mode, before inserting > a new da entry in the extent status tree. > > -ritesh
On 2024/4/26 20:57, Ritesh Harjani (IBM) wrote: > Ritesh Harjani (IBM) <ritesh.list@gmail.com> writes: > >> Zhang Yi <yi.zhang@huaweicloud.com> writes: >> >>> From: Zhang Yi <yi.zhang@huawei.com> >>> >>> Now we lookup extent status entry without holding the i_data_sem before >>> inserting delalloc block, it works fine in buffered write path and >>> because it holds i_rwsem and folio lock, and the mmap path holds folio >>> lock, so the found extent locklessly couldn't be modified concurrently. >>> But it could be raced by fallocate since it allocate block whitout >>> holding i_rwsem and folio lock. >>> >>> ext4_page_mkwrite() ext4_fallocate() >>> block_page_mkwrite() >>> ext4_da_map_blocks() >>> //find hole in extent status tree >>> ext4_alloc_file_blocks() >>> ext4_map_blocks() >>> //allocate block and unwritten extent >>> ext4_insert_delayed_block() >>> ext4_da_reserve_space() >>> //reserve one more block >>> ext4_es_insert_delayed_block() >>> //drop unwritten extent and add delayed extent by mistake >>> >>> Then, the delalloc extent is wrong until writeback, the one more >>> reserved block can't be release any more and trigger below warning: >>> >>> EXT4-fs (pmem2): Inode 13 (00000000bbbd4d23): i_reserved_data_blocks(1) not cleared! >>> >>> Hold i_data_sem in write mode directly can fix the problem, but it's >>> expansive, we should keep the lockless check and check the extent again >>> once we need to add an new delalloc block. >> >> Hi Zhang, >> >> It's a nice finding. I was wondering if this was caught in any of the >> xfstests? >> Hi, Ritesh I caught this issue when I tested my iomap series in generic/344 and generic/346. It's easy to reproduce because the iomap's buffered write path doesn't hold folio lock while inserting delalloc blocks, so it could be raced by the mmap page fault path. But the buffer_head's buffered write path can't trigger this problem, the race between buffered write path and fallocate path was discovered while I was analyzing the code, so I'm not sure if it could be caught by xfstests now, at least I haven't noticed this problem so far. >> I have reworded some of the commit message, feel free to use it if you >> think this version is better. The use of which path uses which locks was >> a bit confusing in the original commit message. >> Thanks for the message improvement, it looks more clear then mine, I will use it. Thanks, Yi. >> <reworded from your original commit msg> >> >> ext4_da_map_blocks(), first looks up the extent status tree for any >> extent entry with i_data_sem held in read mode. It then unlocks >> i_data_sem, if it can't find an entry and take this lock in write >> mode for inserting a new da entry. > > Sorry about this above paragraph. I messed this paragraph. > Here is the correct version of this. > > ext4_da_map_blocks looks up for any extent entry in the extent status > tree (w/o i_data_sem) and then the looks up for any ondisk extent > mapping (with i_data_sem in read mode). > > If it finds a hole in the extent status tree or if it couldn't find any > entry at all, it then takes the i_data_sem in write mode to add a da entry > into the extent status tree. This can actually race with page mkwrite > & fallocate path. > > Note that this is ok between... <and the rest can remain same> > >> >> This is ok between - >> 1. ext4 buffered-write path v/s ext4_page_mkwrite(), because of the >> folio lock >> 2. ext4 buffered write path v/s ext4 fallocate because of the inode >> lock. >> > > >> But this can race between ext4_page_mkwrite() & ext4 fallocate path - >> >> ext4_page_mkwrite() ext4_fallocate() >> block_page_mkwrite() >> ext4_da_map_blocks() >> //find hole in extent status tree >> ext4_alloc_file_blocks() >> ext4_map_blocks() >> //allocate block and unwritten extent >> ext4_insert_delayed_block() >> ext4_da_reserve_space() >> //reserve one more block >> ext4_es_insert_delayed_block() >> //drop unwritten extent and add delayed extent by mistake >> >> Then, the delalloc extent is wrong until writeback and the extra >> reserved block can't be released any more and it triggers below warning: >> >> EXT4-fs (pmem2): Inode 13 (00000000bbbd4d23): i_reserved_data_blocks(1) not cleared! >> >> This patch fixes the problem by looking up extent status tree again >> while the i_data_sem is held in write mode. If it still can't find >> any entry, then we insert a new da entry into the extent status tree. >> >>> >>> Cc: stable@vger.kernel.org >>> Signed-off-by: Zhang Yi <yi.zhang@huawei.com> >>> --- >>> fs/ext4/inode.c | 19 +++++++++++++++++++ >>> 1 file changed, 19 insertions(+) >>> >>> diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c >>> index 6a41172c06e1..118b0497a954 100644 >>> --- a/fs/ext4/inode.c >>> +++ b/fs/ext4/inode.c >>> @@ -1737,6 +1737,7 @@ static int ext4_da_map_blocks(struct inode *inode, sector_t iblock, >>> if (ext4_es_is_hole(&es)) >>> goto add_delayed; >>> >>> +found: >>> /* >>> * Delayed extent could be allocated by fallocate. >>> * So we need to check it. >>> @@ -1781,6 +1782,24 @@ static int ext4_da_map_blocks(struct inode *inode, sector_t iblock, >>> >>> add_delayed: >>> down_write(&EXT4_I(inode)->i_data_sem); >>> + /* >>> + * Lookup extents tree again under i_data_sem, make sure this >>> + * inserting delalloc range haven't been delayed or allocated >>> + * whitout holding i_rwsem and folio lock. >>> + */ >> >> page fault path (ext4_page_mkwrite does not take i_rwsem) and fallocate >> path (no folio lock) can race. Make sure we lookup the extent status >> tree here again while i_data_sem is held in write mode, before inserting >> a new da entry in the extent status tree. >> >> > > > -ritesh >
Zhang Yi <yi.zhang@huaweicloud.com> writes: > On 2024/4/26 20:57, Ritesh Harjani (IBM) wrote: >> Ritesh Harjani (IBM) <ritesh.list@gmail.com> writes: >> >>> Zhang Yi <yi.zhang@huaweicloud.com> writes: >>> >>>> From: Zhang Yi <yi.zhang@huawei.com> >>>> >>>> Now we lookup extent status entry without holding the i_data_sem before >>>> inserting delalloc block, it works fine in buffered write path and >>>> because it holds i_rwsem and folio lock, and the mmap path holds folio >>>> lock, so the found extent locklessly couldn't be modified concurrently. >>>> But it could be raced by fallocate since it allocate block whitout >>>> holding i_rwsem and folio lock. >>>> >>>> ext4_page_mkwrite() ext4_fallocate() >>>> block_page_mkwrite() >>>> ext4_da_map_blocks() >>>> //find hole in extent status tree >>>> ext4_alloc_file_blocks() >>>> ext4_map_blocks() >>>> //allocate block and unwritten extent >>>> ext4_insert_delayed_block() >>>> ext4_da_reserve_space() >>>> //reserve one more block >>>> ext4_es_insert_delayed_block() >>>> //drop unwritten extent and add delayed extent by mistake >>>> >>>> Then, the delalloc extent is wrong until writeback, the one more >>>> reserved block can't be release any more and trigger below warning: >>>> >>>> EXT4-fs (pmem2): Inode 13 (00000000bbbd4d23): i_reserved_data_blocks(1) not cleared! >>>> >>>> Hold i_data_sem in write mode directly can fix the problem, but it's >>>> expansive, we should keep the lockless check and check the extent again >>>> once we need to add an new delalloc block. >>> >>> Hi Zhang, >>> >>> It's a nice finding. I was wondering if this was caught in any of the >>> xfstests? >>> > > Hi, Ritesh > > I caught this issue when I tested my iomap series in generic/344 and > generic/346. It's easy to reproduce because the iomap's buffered write path > doesn't hold folio lock while inserting delalloc blocks, so it could be raced > by the mmap page fault path. But the buffer_head's buffered write path can't > trigger this problem, ya right! That's the difference between how ->map_blocks() is called between buffer_head v/s iomap path. In iomap the ->map_blocks() call happens first to map a large extent and then it iterate over all the locked folios covering the mapped extent for doing writes. Whereas in buffer_head while iterating, we first instantiate/lock the folio and then call ->map_blocks() to map an extent for the given folio. ... So this opens up this window for a race between iomap buffered write path v/s page mkwrite path for inserting delalloc blocks entries. > the race between buffered write path and fallocate path > was discovered while I was analyzing the code, so I'm not sure if it could > be caught by xfstests now, at least I haven't noticed this problem so far. > Did you mean the race between page fault path and fallocate path here? Because buffered write path and fallocate path should not have any race since both takes the inode_lock. I guess you meant page fault path and fallocate path for which you wrote this patch too :) I am surprised, why we cannot see the this race between page mkwrite and fallocate in fstests for inserting da entries to extent status cache. Because the race you identified looks like a legitimate race and is mostly happening since ext4_da_map_blocks() was not doing the right thing. ... looking at the src/holetest, it doesn't really excercise this path. So maybe we can writing such fstest to trigger this race. >>> I have reworded some of the commit message, feel free to use it if you >>> think this version is better. The use of which path uses which locks was >>> a bit confusing in the original commit message. >>> > > Thanks for the message improvement, it looks more clear then mine, I will > use it. > Glad, it was helpful. -ritesh
On 2024/4/27 0:39, Ritesh Harjani (IBM) wrote: > Zhang Yi <yi.zhang@huaweicloud.com> writes: > >> On 2024/4/26 20:57, Ritesh Harjani (IBM) wrote: >>> Ritesh Harjani (IBM) <ritesh.list@gmail.com> writes: >>> >>>> Zhang Yi <yi.zhang@huaweicloud.com> writes: >>>> >>>>> From: Zhang Yi <yi.zhang@huawei.com> >>>>> >>>>> Now we lookup extent status entry without holding the i_data_sem before >>>>> inserting delalloc block, it works fine in buffered write path and >>>>> because it holds i_rwsem and folio lock, and the mmap path holds folio >>>>> lock, so the found extent locklessly couldn't be modified concurrently. >>>>> But it could be raced by fallocate since it allocate block whitout >>>>> holding i_rwsem and folio lock. >>>>> >>>>> ext4_page_mkwrite() ext4_fallocate() >>>>> block_page_mkwrite() >>>>> ext4_da_map_blocks() >>>>> //find hole in extent status tree >>>>> ext4_alloc_file_blocks() >>>>> ext4_map_blocks() >>>>> //allocate block and unwritten extent >>>>> ext4_insert_delayed_block() >>>>> ext4_da_reserve_space() >>>>> //reserve one more block >>>>> ext4_es_insert_delayed_block() >>>>> //drop unwritten extent and add delayed extent by mistake >>>>> >>>>> Then, the delalloc extent is wrong until writeback, the one more >>>>> reserved block can't be release any more and trigger below warning: >>>>> >>>>> EXT4-fs (pmem2): Inode 13 (00000000bbbd4d23): i_reserved_data_blocks(1) not cleared! >>>>> >>>>> Hold i_data_sem in write mode directly can fix the problem, but it's >>>>> expansive, we should keep the lockless check and check the extent again >>>>> once we need to add an new delalloc block. >>>> >>>> Hi Zhang, >>>> >>>> It's a nice finding. I was wondering if this was caught in any of the >>>> xfstests? >>>> >> >> Hi, Ritesh >> >> I caught this issue when I tested my iomap series in generic/344 and >> generic/346. It's easy to reproduce because the iomap's buffered write path >> doesn't hold folio lock while inserting delalloc blocks, so it could be raced >> by the mmap page fault path. But the buffer_head's buffered write path can't >> trigger this problem, > > ya right! That's the difference between how ->map_blocks() is called > between buffer_head v/s iomap path. In iomap the ->map_blocks() call > happens first to map a large extent and then it iterate over all the > locked folios covering the mapped extent for doing writes. > Whereas in buffer_head while iterating, we first instantiate/lock the > folio and then call ->map_blocks() to map an extent for the given folio. > > ... So this opens up this window for a race between iomap buffered write > path v/s page mkwrite path for inserting delalloc blocks entries. > >> the race between buffered write path and fallocate path >> was discovered while I was analyzing the code, so I'm not sure if it could >> be caught by xfstests now, at least I haven't noticed this problem so far. >> > > Did you mean the race between page fault path and fallocate path here? > Because buffered write path and fallocate path should not have any race > since both takes the inode_lock. I guess you meant page fault path and > fallocate path for which you wrote this patch too :) Yep. > > I am surprised, why we cannot see the this race between page mkwrite and > fallocate in fstests for inserting da entries to extent status cache. > Because the race you identified looks like a legitimate race and is > mostly happening since ext4_da_map_blocks() was not doing the right > thing. > ... looking at the src/holetest, it doesn't really excercise this path. > So maybe we can writing such fstest to trigger this race. > I guess the stress tests and smoke tests in fstests have caught it, e.g. generic/476. Since there is only one error message in ext4_destroy_inode() when the race issue happened, we can't detect it unless we go and check the logs manually. I suppose we need to add more warnings, something like this, how does it sound? diff --git a/fs/ext4/super.c b/fs/ext4/super.c index c8b691e605f1..4b6fd9b63b12 100644 --- a/fs/ext4/super.c +++ b/fs/ext4/super.c @@ -1255,6 +1255,8 @@ static void ext4_percpu_param_destroy(struct ext4_sb_info *sbi) percpu_counter_destroy(&sbi->s_freeclusters_counter); percpu_counter_destroy(&sbi->s_freeinodes_counter); percpu_counter_destroy(&sbi->s_dirs_counter); + WARN_ON_ONCE(!ext4_forced_shutdown(sbi->s_sb) && + percpu_counter_sum(&sbi->s_dirtyclusters_counter)); percpu_counter_destroy(&sbi->s_dirtyclusters_counter); percpu_counter_destroy(&sbi->s_sra_exceeded_retry_limit); percpu_free_rwsem(&sbi->s_writepages_rwsem); @@ -1476,7 +1478,8 @@ static void ext4_destroy_inode(struct inode *inode) dump_stack(); } - if (EXT4_I(inode)->i_reserved_data_blocks) + if (!ext4_forced_shutdown(inode->i_sb) && + WARN_ON_ONCE(EXT4_I(inode)->i_reserved_data_blocks)) ext4_msg(inode->i_sb, KERN_ERR, "Inode %lu (%p): i_reserved_data_blocks (%u) not cleared!", inode->i_ino, EXT4_I(inode), Thanks, Yi. > >>>> I have reworded some of the commit message, feel free to use it if you >>>> think this version is better. The use of which path uses which locks was >>>> a bit confusing in the original commit message. >>>> >> >> Thanks for the message improvement, it looks more clear then mine, I will >> use it. >> > > Glad, it was helpful. > > -ritesh >
Zhang Yi <yi.zhang@huaweicloud.com> writes: > On 2024/4/27 0:39, Ritesh Harjani (IBM) wrote: >> Zhang Yi <yi.zhang@huaweicloud.com> writes: >> >>> On 2024/4/26 20:57, Ritesh Harjani (IBM) wrote: >>>> Ritesh Harjani (IBM) <ritesh.list@gmail.com> writes: >>>> >>>>> Zhang Yi <yi.zhang@huaweicloud.com> writes: >>>>> >>>>>> From: Zhang Yi <yi.zhang@huawei.com> >>>>>> >>>>>> Now we lookup extent status entry without holding the i_data_sem before >>>>>> inserting delalloc block, it works fine in buffered write path and >>>>>> because it holds i_rwsem and folio lock, and the mmap path holds folio >>>>>> lock, so the found extent locklessly couldn't be modified concurrently. >>>>>> But it could be raced by fallocate since it allocate block whitout >>>>>> holding i_rwsem and folio lock. >>>>>> >>>>>> ext4_page_mkwrite() ext4_fallocate() >>>>>> block_page_mkwrite() >>>>>> ext4_da_map_blocks() >>>>>> //find hole in extent status tree >>>>>> ext4_alloc_file_blocks() >>>>>> ext4_map_blocks() >>>>>> //allocate block and unwritten extent >>>>>> ext4_insert_delayed_block() >>>>>> ext4_da_reserve_space() >>>>>> //reserve one more block >>>>>> ext4_es_insert_delayed_block() >>>>>> //drop unwritten extent and add delayed extent by mistake >>>>>> >>>>>> Then, the delalloc extent is wrong until writeback, the one more >>>>>> reserved block can't be release any more and trigger below warning: >>>>>> >>>>>> EXT4-fs (pmem2): Inode 13 (00000000bbbd4d23): i_reserved_data_blocks(1) not cleared! >>>>>> >>>>>> Hold i_data_sem in write mode directly can fix the problem, but it's >>>>>> expansive, we should keep the lockless check and check the extent again >>>>>> once we need to add an new delalloc block. >>>>> >>>>> Hi Zhang, >>>>> >>>>> It's a nice finding. I was wondering if this was caught in any of the >>>>> xfstests? >>>>> >>> >>> Hi, Ritesh >>> >>> I caught this issue when I tested my iomap series in generic/344 and >>> generic/346. It's easy to reproduce because the iomap's buffered write path >>> doesn't hold folio lock while inserting delalloc blocks, so it could be raced >>> by the mmap page fault path. But the buffer_head's buffered write path can't >>> trigger this problem, >> >> ya right! That's the difference between how ->map_blocks() is called >> between buffer_head v/s iomap path. In iomap the ->map_blocks() call >> happens first to map a large extent and then it iterate over all the >> locked folios covering the mapped extent for doing writes. >> Whereas in buffer_head while iterating, we first instantiate/lock the >> folio and then call ->map_blocks() to map an extent for the given folio. >> >> ... So this opens up this window for a race between iomap buffered write >> path v/s page mkwrite path for inserting delalloc blocks entries. >> >>> the race between buffered write path and fallocate path >>> was discovered while I was analyzing the code, so I'm not sure if it could >>> be caught by xfstests now, at least I haven't noticed this problem so far. >>> >> >> Did you mean the race between page fault path and fallocate path here? >> Because buffered write path and fallocate path should not have any race >> since both takes the inode_lock. I guess you meant page fault path and >> fallocate path for which you wrote this patch too :) > > Yep. > >> >> I am surprised, why we cannot see the this race between page mkwrite and >> fallocate in fstests for inserting da entries to extent status cache. >> Because the race you identified looks like a legitimate race and is >> mostly happening since ext4_da_map_blocks() was not doing the right >> thing. >> ... looking at the src/holetest, it doesn't really excercise this path. >> So maybe we can writing such fstest to trigger this race. >> > > I guess the stress tests and smoke tests in fstests have caught it, > e.g. generic/476. Since there is only one error message in ext4_destroy_inode() > when the race issue happened, we can't detect it unless we go and check the logs > manually. Hi Zhang, I wasn't able to reproduce the any error messages with generic/476. > > I suppose we need to add more warnings, something like this, how does it sound? > > diff --git a/fs/ext4/super.c b/fs/ext4/super.c > index c8b691e605f1..4b6fd9b63b12 100644 > --- a/fs/ext4/super.c > +++ b/fs/ext4/super.c > @@ -1255,6 +1255,8 @@ static void ext4_percpu_param_destroy(struct ext4_sb_info *sbi) > percpu_counter_destroy(&sbi->s_freeclusters_counter); > percpu_counter_destroy(&sbi->s_freeinodes_counter); > percpu_counter_destroy(&sbi->s_dirs_counter); > + WARN_ON_ONCE(!ext4_forced_shutdown(sbi->s_sb) && > + percpu_counter_sum(&sbi->s_dirtyclusters_counter)); > percpu_counter_destroy(&sbi->s_dirtyclusters_counter); > percpu_counter_destroy(&sbi->s_sra_exceeded_retry_limit); > percpu_free_rwsem(&sbi->s_writepages_rwsem); > @@ -1476,7 +1478,8 @@ static void ext4_destroy_inode(struct inode *inode) > dump_stack(); > } > > - if (EXT4_I(inode)->i_reserved_data_blocks) > + if (!ext4_forced_shutdown(inode->i_sb) && > + WARN_ON_ONCE(EXT4_I(inode)->i_reserved_data_blocks)) > ext4_msg(inode->i_sb, KERN_ERR, > "Inode %lu (%p): i_reserved_data_blocks (%u) not cleared!", > inode->i_ino, EXT4_I(inode), > I also ran ext4 -g auto and I couldn't reproduce anything with above patch. Please note that I didn't use this patch series for testing. I was running xfstests on upstream kernel with above diff (because that's what the idea was that the problem even exists in upstream kernel and are we able to observe the race with page mkwrite and fallocate path) -ritesh > > Thanks, > Yi. > >> >>>>> I have reworded some of the commit message, feel free to use it if you >>>>> think this version is better. The use of which path uses which locks was >>>>> a bit confusing in the original commit message. >>>>> >>> >>> Thanks for the message improvement, it looks more clear then mine, I will >>> use it. >>> >> >> Glad, it was helpful. >> >> -ritesh >>
diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c index 6a41172c06e1..118b0497a954 100644 --- a/fs/ext4/inode.c +++ b/fs/ext4/inode.c @@ -1737,6 +1737,7 @@ static int ext4_da_map_blocks(struct inode *inode, sector_t iblock, if (ext4_es_is_hole(&es)) goto add_delayed; +found: /* * Delayed extent could be allocated by fallocate. * So we need to check it. @@ -1781,6 +1782,24 @@ static int ext4_da_map_blocks(struct inode *inode, sector_t iblock, add_delayed: down_write(&EXT4_I(inode)->i_data_sem); + /* + * Lookup extents tree again under i_data_sem, make sure this + * inserting delalloc range haven't been delayed or allocated + * whitout holding i_rwsem and folio lock. + */ + if (ext4_es_lookup_extent(inode, iblock, NULL, &es)) { + if (!ext4_es_is_hole(&es)) { + up_write(&EXT4_I(inode)->i_data_sem); + goto found; + } + } else if (!ext4_has_inline_data(inode)) { + retval = ext4_map_query_blocks(NULL, inode, map); + if (retval) { + up_write(&EXT4_I(inode)->i_data_sem); + return retval; + } + } + retval = ext4_insert_delayed_block(inode, map->m_lblk); up_write(&EXT4_I(inode)->i_data_sem); if (retval)