| Message ID | 494196DD.5070600@cosmosbay.com |
|---|---|
| State | Not Applicable, archived |
| Delegated to: | David Miller |
| Headers | show |
On Friday 12 December 2008 09:40, Eric Dumazet wrote: > From: Christoph Lameter <cl@linux-foundation.org> > > [PATCH] fs: struct file move from call_rcu() to SLAB_DESTROY_BY_RCU > > Currently we schedule RCU frees for each file we free separately. That has > several drawbacks against the earlier file handling (in 2.6.5 f.e.), which > did not require RCU callbacks: > > 1. Excessive number of RCU callbacks can be generated causing long RCU > queues that in turn cause long latencies. We hit SLUB page allocation > more often than necessary. > > 2. The cache hot object is not preserved between free and realloc. A close > followed by another open is very fast with the RCUless approach because > the last freed object is returned by the slab allocator that is > still cache hot. RCU free means that the object is not immediately > available again. The new object is cache cold and therefore open/close > performance tests show a significant degradation with the RCU > implementation. > > One solution to this problem is to move the RCU freeing into the Slab > allocator by specifying SLAB_DESTROY_BY_RCU as an option at slab creation > time. The slab allocator will do RCU frees only when it is necessary > to dispose of slabs of objects (rare). So with that approach we can cut > out the RCU overhead significantly. > > However, the slab allocator may return the object for another use even > before the RCU period has expired under SLAB_DESTROY_BY_RCU. This means > there is the (unlikely) possibility that the object is going to be > switched under us in sections protected by rcu_read_lock() and > rcu_read_unlock(). So we need to verify that we have acquired the correct > object after establishing a stable object reference (incrementing the > refcounter does that). > > > Signed-off-by: Christoph Lameter <cl@linux-foundation.org> > Signed-off-by: Eric Dumazet <dada1@cosmosbay.com> > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> > --- > Documentation/filesystems/files.txt | 21 ++++++++++++++-- > fs/file_table.c | 33 ++++++++++++++++++-------- > include/linux/fs.h | 5 --- > 3 files changed, 42 insertions(+), 17 deletions(-) > > diff --git a/Documentation/filesystems/files.txt > b/Documentation/filesystems/files.txt index ac2facc..6916baa 100644 > --- a/Documentation/filesystems/files.txt > +++ b/Documentation/filesystems/files.txt > @@ -78,13 +78,28 @@ the fdtable structure - > that look-up may race with the last put() operation on the > file structure. This is avoided using atomic_long_inc_not_zero() > on ->f_count : > + As file structures are allocated with SLAB_DESTROY_BY_RCU, > + they can also be freed before a RCU grace period, and reused, > + but still as a struct file. > + It is necessary to check again after getting > + a stable reference (ie after atomic_long_inc_not_zero()), > + that fcheck_files(files, fd) points to the same file. > > rcu_read_lock(); > file = fcheck_files(files, fd); > if (file) { > - if (atomic_long_inc_not_zero(&file->f_count)) > + if (atomic_long_inc_not_zero(&file->f_count)) { > *fput_needed = 1; > - else > + /* > + * Now we have a stable reference to an object. > + * Check if other threads freed file and reallocated it. > + */ > + if (file != fcheck_files(files, fd)) { > + *fput_needed = 0; > + put_filp(file); > + file = NULL; > + } > + } else > /* Didn't get the reference, someone's freed */ > file = NULL; > } > @@ -95,6 +110,8 @@ the fdtable structure - > atomic_long_inc_not_zero() detects if refcounts is already zero or > goes to zero during increment. If it does, we fail > fget()/fget_light(). > + The second call to fcheck_files(files, fd) checks that this filp > + was not freed, then reused by an other thread. > > 6. Since both fdtable and file structures can be looked up > lock-free, they must be installed using rcu_assign_pointer() > diff --git a/fs/file_table.c b/fs/file_table.c > index a46e880..3e9259d 100644 > --- a/fs/file_table.c > +++ b/fs/file_table.c > @@ -37,17 +37,11 @@ static struct kmem_cache *filp_cachep __read_mostly; > > static struct percpu_counter nr_files __cacheline_aligned_in_smp; > > -static inline void file_free_rcu(struct rcu_head *head) > -{ > - struct file *f = container_of(head, struct file, f_u.fu_rcuhead); > - kmem_cache_free(filp_cachep, f); > -} > - > static inline void file_free(struct file *f) > { > percpu_counter_dec(&nr_files); > file_check_state(f); > - call_rcu(&f->f_u.fu_rcuhead, file_free_rcu); > + kmem_cache_free(filp_cachep, f); > } > > /* > @@ -306,6 +300,14 @@ struct file *fget(unsigned int fd) > rcu_read_unlock(); > return NULL; > } > + /* > + * Now we have a stable reference to an object. > + * Check if other threads freed file and re-allocated it. > + */ > + if (unlikely(file != fcheck_files(files, fd))) { > + put_filp(file); > + file = NULL; > + } This is a non-trivial change, because that put_filp may drop the last reference to the file. So now we have the case where we free the file from a context in which it had never been allocated. From a quick glance though the callchains, I can't seen an obvious problem. But it needs to have documentation in put_filp, or at least a mention in the changelog, and also cc'ed to the security lists. Also, it adds code and cost to the get/put path in return for improvement in the free path. get/put is the more common path, but it is a small loss for a big improvement. So it might be worth it. But it is not justified by your microbenchmark. Do we have a more useful case that it helps? -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
On Tuesday 24 July 2007 11:13, Nick Piggin wrote: > On Friday 12 December 2008 09:40, Eric Dumazet wrote: > > From: Christoph Lameter <cl@linux-foundation.org> > > > > [PATCH] fs: struct file move from call_rcu() to SLAB_DESTROY_BY_RCU > > > > Currently we schedule RCU frees for each file we free separately. That > > has several drawbacks against the earlier file handling (in 2.6.5 f.e.), > > which did not require RCU callbacks: > > > > 1. Excessive number of RCU callbacks can be generated causing long RCU > > queues that in turn cause long latencies. We hit SLUB page allocation > > more often than necessary. > > > > 2. The cache hot object is not preserved between free and realloc. A > > close followed by another open is very fast with the RCUless approach > > because the last freed object is returned by the slab allocator that is > > still cache hot. RCU free means that the object is not immediately > > available again. The new object is cache cold and therefore open/close > > performance tests show a significant degradation with the RCU > > implementation. > > > > One solution to this problem is to move the RCU freeing into the Slab > > allocator by specifying SLAB_DESTROY_BY_RCU as an option at slab creation > > time. The slab allocator will do RCU frees only when it is necessary > > to dispose of slabs of objects (rare). So with that approach we can cut > > out the RCU overhead significantly. > > > > However, the slab allocator may return the object for another use even > > before the RCU period has expired under SLAB_DESTROY_BY_RCU. This means > > there is the (unlikely) possibility that the object is going to be > > switched under us in sections protected by rcu_read_lock() and > > rcu_read_unlock(). So we need to verify that we have acquired the correct > > object after establishing a stable object reference (incrementing the > > refcounter does that). > > > > > > Signed-off-by: Christoph Lameter <cl@linux-foundation.org> > > Signed-off-by: Eric Dumazet <dada1@cosmosbay.com> > > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> > > --- > > Documentation/filesystems/files.txt | 21 ++++++++++++++-- > > fs/file_table.c | 33 ++++++++++++++++++-------- > > include/linux/fs.h | 5 --- > > 3 files changed, 42 insertions(+), 17 deletions(-) > > > > diff --git a/Documentation/filesystems/files.txt > > b/Documentation/filesystems/files.txt index ac2facc..6916baa 100644 > > --- a/Documentation/filesystems/files.txt > > +++ b/Documentation/filesystems/files.txt > > @@ -78,13 +78,28 @@ the fdtable structure - > > that look-up may race with the last put() operation on the > > file structure. This is avoided using atomic_long_inc_not_zero() > > on ->f_count : > > + As file structures are allocated with SLAB_DESTROY_BY_RCU, > > + they can also be freed before a RCU grace period, and reused, > > + but still as a struct file. > > + It is necessary to check again after getting > > + a stable reference (ie after atomic_long_inc_not_zero()), > > + that fcheck_files(files, fd) points to the same file. > > > > rcu_read_lock(); > > file = fcheck_files(files, fd); > > if (file) { > > - if (atomic_long_inc_not_zero(&file->f_count)) > > + if (atomic_long_inc_not_zero(&file->f_count)) { > > *fput_needed = 1; > > - else > > + /* > > + * Now we have a stable reference to an object. > > + * Check if other threads freed file and reallocated it. > > + */ > > + if (file != fcheck_files(files, fd)) { > > + *fput_needed = 0; > > + put_filp(file); > > + file = NULL; > > + } > > + } else > > /* Didn't get the reference, someone's freed */ > > file = NULL; > > } > > @@ -95,6 +110,8 @@ the fdtable structure - > > atomic_long_inc_not_zero() detects if refcounts is already zero or > > goes to zero during increment. If it does, we fail > > fget()/fget_light(). > > + The second call to fcheck_files(files, fd) checks that this filp > > + was not freed, then reused by an other thread. > > > > 6. Since both fdtable and file structures can be looked up > > lock-free, they must be installed using rcu_assign_pointer() > > diff --git a/fs/file_table.c b/fs/file_table.c > > index a46e880..3e9259d 100644 > > --- a/fs/file_table.c > > +++ b/fs/file_table.c > > @@ -37,17 +37,11 @@ static struct kmem_cache *filp_cachep __read_mostly; > > > > static struct percpu_counter nr_files __cacheline_aligned_in_smp; > > > > -static inline void file_free_rcu(struct rcu_head *head) > > -{ > > - struct file *f = container_of(head, struct file, f_u.fu_rcuhead); > > - kmem_cache_free(filp_cachep, f); > > -} > > - > > static inline void file_free(struct file *f) > > { > > percpu_counter_dec(&nr_files); > > file_check_state(f); > > - call_rcu(&f->f_u.fu_rcuhead, file_free_rcu); > > + kmem_cache_free(filp_cachep, f); > > } > > > > /* > > @@ -306,6 +300,14 @@ struct file *fget(unsigned int fd) > > rcu_read_unlock(); > > return NULL; > > } > > + /* > > + * Now we have a stable reference to an object. > > + * Check if other threads freed file and re-allocated it. > > + */ > > + if (unlikely(file != fcheck_files(files, fd))) { > > + put_filp(file); > > + file = NULL; > > + } > > This is a non-trivial change, because that put_filp may drop the last > reference to the file. So now we have the case where we free the file > from a context in which it had never been allocated. > > From a quick glance though the callchains, I can't seen an obvious > problem. But it needs to have documentation in put_filp, or at least > a mention in the changelog, and also cc'ed to the security lists. > > Also, it adds code and cost to the get/put path in return for > improvement in the free path. get/put is the more common path, but > it is a small loss for a big improvement. So it might be worth it. But > it is not justified by your microbenchmark. Do we have a more useful > case that it helps? Sorry, my clock screwed up and I didn't notice :( -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
diff --git a/Documentation/filesystems/files.txt b/Documentation/filesystems/files.txt index ac2facc..6916baa 100644 --- a/Documentation/filesystems/files.txt +++ b/Documentation/filesystems/files.txt @@ -78,13 +78,28 @@ the fdtable structure - that look-up may race with the last put() operation on the file structure. This is avoided using atomic_long_inc_not_zero() on ->f_count : + As file structures are allocated with SLAB_DESTROY_BY_RCU, + they can also be freed before a RCU grace period, and reused, + but still as a struct file. + It is necessary to check again after getting + a stable reference (ie after atomic_long_inc_not_zero()), + that fcheck_files(files, fd) points to the same file. rcu_read_lock(); file = fcheck_files(files, fd); if (file) { - if (atomic_long_inc_not_zero(&file->f_count)) + if (atomic_long_inc_not_zero(&file->f_count)) { *fput_needed = 1; - else + /* + * Now we have a stable reference to an object. + * Check if other threads freed file and reallocated it. + */ + if (file != fcheck_files(files, fd)) { + *fput_needed = 0; + put_filp(file); + file = NULL; + } + } else /* Didn't get the reference, someone's freed */ file = NULL; } @@ -95,6 +110,8 @@ the fdtable structure - atomic_long_inc_not_zero() detects if refcounts is already zero or goes to zero during increment. If it does, we fail fget()/fget_light(). + The second call to fcheck_files(files, fd) checks that this filp + was not freed, then reused by an other thread. 6. Since both fdtable and file structures can be looked up lock-free, they must be installed using rcu_assign_pointer() diff --git a/fs/file_table.c b/fs/file_table.c index a46e880..3e9259d 100644 --- a/fs/file_table.c +++ b/fs/file_table.c @@ -37,17 +37,11 @@ static struct kmem_cache *filp_cachep __read_mostly; static struct percpu_counter nr_files __cacheline_aligned_in_smp; -static inline void file_free_rcu(struct rcu_head *head) -{ - struct file *f = container_of(head, struct file, f_u.fu_rcuhead); - kmem_cache_free(filp_cachep, f); -} - static inline void file_free(struct file *f) { percpu_counter_dec(&nr_files); file_check_state(f); - call_rcu(&f->f_u.fu_rcuhead, file_free_rcu); + kmem_cache_free(filp_cachep, f); } /* @@ -306,6 +300,14 @@ struct file *fget(unsigned int fd) rcu_read_unlock(); return NULL; } + /* + * Now we have a stable reference to an object. + * Check if other threads freed file and re-allocated it. + */ + if (unlikely(file != fcheck_files(files, fd))) { + put_filp(file); + file = NULL; + } } rcu_read_unlock(); @@ -333,9 +335,19 @@ struct file *fget_light(unsigned int fd, int *fput_needed) rcu_read_lock(); file = fcheck_files(files, fd); if (file) { - if (atomic_long_inc_not_zero(&file->f_count)) + if (atomic_long_inc_not_zero(&file->f_count)) { *fput_needed = 1; - else + /* + * Now we have a stable reference to an object. + * Check if other threads freed this file and + * re-allocated it. + */ + if (unlikely(file != fcheck_files(files, fd))) { + *fput_needed = 0; + put_filp(file); + file = NULL; + } + } else /* Didn't get the reference, someone's freed */ file = NULL; } @@ -402,7 +414,8 @@ void __init files_init(unsigned long mempages) int n; filp_cachep = kmem_cache_create("filp", sizeof(struct file), 0, - SLAB_HWCACHE_ALIGN | SLAB_PANIC, NULL); + SLAB_HWCACHE_ALIGN | SLAB_DESTROY_BY_RCU | SLAB_PANIC, + NULL); /* * One file with associated inode and dcache is very roughly 1K. diff --git a/include/linux/fs.h b/include/linux/fs.h index a702d81..a1f56d4 100644 --- a/include/linux/fs.h +++ b/include/linux/fs.h @@ -811,13 +811,8 @@ static inline int ra_has_index(struct file_ra_state *ra, pgoff_t index) #define FILE_MNT_WRITE_RELEASED 2 struct file { - /* - * fu_list becomes invalid after file_free is called and queued via - * fu_rcuhead for RCU freeing - */ union { struct list_head fu_list; - struct rcu_head fu_rcuhead; } f_u; struct path f_path; #define f_dentry f_path.dentry