| Message ID | 87k32uf5zs.fsf@blackfin.pond.sub.org |
|---|---|
| State | New |
| Headers | show |
On 2014-11-17 at 11:58, Markus Armbruster wrote: > Max Reitz <mreitz@redhat.com> writes: > >> On 2014-11-17 at 11:18, Markus Armbruster wrote: >>> Commit 5500316 (May 2012) implemented raw_co_is_allocated() as >>> follows: >>> >>> 1. If defined(CONFIG_FIEMAP), use the FS_IOC_FIEMAP ioctl >>> >>> 2. Else if defined(SEEK_HOLE) && defined(SEEK_DATA), use lseek() >>> >>> 3. Else pretend there are no holes >>> >>> Later on, raw_co_is_allocated() was generalized to >>> raw_co_get_block_status(). >>> >>> Commit 4f11aa8 (May 2014) changed it to try the three methods in order >>> until success, because "there may be implementations which support >>> [SEEK_HOLE/SEEK_DATA] but not [FIEMAP] (e.g., NFSv4.2) as well as vice >>> versa." >>> >>> Unfortunately, we used FIEMAP incorrectly: we lacked FIEMAP_FLAG_SYNC. >>> Commit 38c4d0a (Sep 2014) added it. Because that's a significant >>> speed hit, the next commit 7c159037 put SEEK_HOLE/SEEK_DATA first. >>> >>> As you see, the obvious use of FIEMAP is wrong, and the correct use is >>> slow. I guess this puts it somewhere between -7 "The obvious use is >>> wrong" and -10 "It's impossible to get right" on Rusty Russel's Hard >>> to Misuse scale[*]. >>> >>> "Fortunately", the FIEMAP code is used only when >>> >>> * SEEK_HOLE/SEEK_DATA aren't defined, but CONFIG_FIEMAP is >>> >>> Uncommon. SEEK_HOLE had no XFS implementation between 2011 (when it >>> was introduced for ext4 and btrfs) and 2012. >>> >>> * SEEK_HOLE/SEEK_DATA and CONFIG_FIEMAP are defined, but lseek() fails >>> >>> Unlikely. >>> >>> Thus, the FIEMAP code executes rarely. Makes it a nice hidey-hole for >>> bugs. Worse, bugs hiding there can theoretically bite even on a host >>> that has SEEK_HOLE/SEEK_DATA. >>> >>> I don't want to worry about this crap, not even theoretically. Get >>> rid of it. >>> >>> [*] http://ozlabs.org/~rusty/index.cgi/tech/2008-04-01.html >>> >>> Signed-off-by: Markus Armbruster <armbru@redhat.com> >>> Reviewed-by: Max Reitz <mreitz@redhat.com> >>> Reviewed-by: Eric Blake <eblake@redhat.com> >>> --- >>> block/raw-posix.c | 60 ++++--------------------------------------------------- >>> 1 file changed, 4 insertions(+), 56 deletions(-) >> I only just now realized that you're not getting rid of FIEMAP >> completely; there's still the skip_fiemap flag in BDRVRawState. I >> don't care for now, though, thus my R-b stands. > You're right! The appended patch should be squashed in, either on > commit, or in a respin. > > > diff --git a/block/raw-posix.c b/block/raw-posix.c > index 0b5d5a8..414e6d1 100644 > --- a/block/raw-posix.c > +++ b/block/raw-posix.c > @@ -148,9 +148,6 @@ typedef struct BDRVRawState { > bool has_write_zeroes:1; > bool discard_zeroes:1; > bool needs_alignment; > -#ifdef CONFIG_FIEMAP > - bool skip_fiemap; > -#endif > } BDRVRawState; > > typedef struct BDRVRawReopenState { With or without thank hunk (better with): Reviewed-by: Max Reitz <mreitz@redhat.com>
On 11/17/2014 03:58 AM, Markus Armbruster wrote: > Max Reitz <mreitz@redhat.com> writes: > >> On 2014-11-17 at 11:18, Markus Armbruster wrote: >>> Commit 5500316 (May 2012) implemented raw_co_is_allocated() as >>> follows: >>> >>> Signed-off-by: Markus Armbruster <armbru@redhat.com> >>> Reviewed-by: Max Reitz <mreitz@redhat.com> >>> Reviewed-by: Eric Blake <eblake@redhat.com> >>> --- >>> block/raw-posix.c | 60 ++++--------------------------------------------------- >>> 1 file changed, 4 insertions(+), 56 deletions(-) >> >> I only just now realized that you're not getting rid of FIEMAP >> completely; there's still the skip_fiemap flag in BDRVRawState. I >> don't care for now, though, thus my R-b stands. > > You're right! The appended patch should be squashed in, either on > commit, or in a respin. My R-b stands whether or not you squash this in; best situation would be squashing this in during commit. > > > diff --git a/block/raw-posix.c b/block/raw-posix.c > index 0b5d5a8..414e6d1 100644 > --- a/block/raw-posix.c > +++ b/block/raw-posix.c > @@ -148,9 +148,6 @@ typedef struct BDRVRawState { > bool has_write_zeroes:1; > bool discard_zeroes:1; > bool needs_alignment; > -#ifdef CONFIG_FIEMAP > - bool skip_fiemap; > -#endif > } BDRVRawState; > > typedef struct BDRVRawReopenState { > > >
diff --git a/block/raw-posix.c b/block/raw-posix.c index 0b5d5a8..414e6d1 100644 --- a/block/raw-posix.c +++ b/block/raw-posix.c @@ -148,9 +148,6 @@ typedef struct BDRVRawState { bool has_write_zeroes:1; bool discard_zeroes:1; bool needs_alignment; -#ifdef CONFIG_FIEMAP - bool skip_fiemap; -#endif } BDRVRawState; typedef struct BDRVRawReopenState {