On 2019/10/23 下午11:41, Nikolay Borisov wrote: > > > On 23.10.19 г. 16:57 ч., Qu Wenruo wrote: >> [BUG] >> When deleting large files (which cross block group boundary) with discard >> mount option, we find some btrfs_discard_extent() calls only trimmed part >> of its space, not the whole range: >> >> btrfs_discard_extent: type=0x1 start=19626196992 len=2144530432 trimmed=1073741824 ratio=50% >> >> type: bbio->map_type, in above case, it's SINGLE DATA. >> start: Logical address of this trim >> len: Logical length of this trim >> trimmed: Physically trimmed bytes >> ratio: trimmed / len >> >> Thus leading some unused space not discarded. >> >> [CAUSE] >> When discard mount option is specified, after a transaction is fully >> committed (super block written to disk), we begin to cleanup pinned >> extents in the following call chain: >> >> btrfs_commit_transaction() >> |- write_all_supers() > > You can remove write_all_supers > >> |- btrfs_finish_extent_commit() >> |- find_first_extent_bit(unpin, 0, &start, &end, EXTENT_DIRTY); >> |- btrfs_discard_extent() >> >> However pinned extents are recorded in an extent_io_tree, which can >> merge adjacent extent states. >> >> When a large file get deleted and it has adjacent file extents across >> block group boundary, we will get a large merged range. > > This is wrong, it will only get merged if the extent spans contiguous bg boundaries > (this is very important!) Yep, skipped some details as I thought it was too obvious, but indeed it needs extra wording. > >> >> Then when we pass the large range into btrfs_discard_extent(), >> btrfs_discard_extent() will just trim the first part, without trimming >> the remaining part. > > Here is what my testing shows: > > mkfs.btrfs -f /dev/vdc > > mount -onodatasum,nospace_cache /dev/vdc /media/scratch/ nospace_cache is important as v1 space cache exists as regular file, which could take up data space and screw up extent layout. (And in original report, v1 cache is the cause of randomness in reproducibility) > xfs_io -f -c "pwrite 0 800m" /media/scratch/file1 && sync > xfs_io -f -c "pwrite 0 300m" /media/scratch/file2 && sync > umount /media/scratch > > mount -odiscard /dev/vdc /media/scratch > rm -f /media/scratch/file2 && sync > trace-cmd show > > umount /media/scratch > > The output I get in trace-cmd is: > > sync-1014 [001] .... 534.272310: btrfs_finish_extent_commit: Discarding 1943011328-2077229055 (len: 134217728) > sync-1014 [001] .... 534.272315: btrfs_discard_extent: Requested to discard: 134217728 but discarded: 134217728 > > sync-1014 [001] .... 534.272325: btrfs_finish_extent_commit: Discarding 2177892352-2358247423 (len: 180355072) > sync-1014 [001] .... 534.272330: btrfs_discard_extent: Requested to discard: 180355072 but discarded: 180355072 > > The extents of this file look like this in the extent tree prior to the trim: > > item 18 key (1943011328 EXTENT_ITEM 134217728) itemoff 15523 itemsize 53 > refs 1 gen 7 flags DATA > extent data backref root FS_TREE objectid 258 offset 0 count 1 > item 19 key (2177892352 EXTENT_ITEM 134217728) itemoff 15470 itemsize 53 > refs 1 gen 7 flags DATA > extent data backref root FS_TREE objectid 258 offset 134217728 count 1 > item 20 key (2177892352 BLOCK_GROUP_ITEM 1073741824) itemoff 15446 itemsize 24 > block group used 180355072 chunk_objectid 256 flags DATA > item 21 key (2312110080 EXTENT_ITEM 46137344) itemoff 15393 itemsize 53 > refs 1 gen 7 flags DATA > extent data backref root FS_TREE objectid 258 offset 268435456 count 1 > > So we have 3 extents 1 of which is in bg 1 and the other 2 in bg2. The 2 extents in bg2 are merged but > since the 2nd bg is not contiguous to the first hence no merging. > > Here comes the requirement why the bg must be contiguous. > > If I modify my test case with slightly different write offsets such that bg1 > is indeed filled and the next extent gets allocated to in bg2, which is adjacent then > the bug is reproduced: > > mkfs.btrfs -f /dev/vdc > > mount -onodatasum,nospace_cache /dev/vdc /media/scratch/ > xfs_io -f -c "pwrite 0 800m" /media/scratch/file1 && sync > xfs_io -f -c "pwrite 0 224m" /media/scratch/file2 && sync > xfs_io -f -c "pwrite 224m 76m" /media/scratch/file2 && sync > umount /media/scratch > > mount -odiscard /dev/vdc /media/scratch > rm -f /media/scratch/file2 && sync > trace-cmd show > > umount /media/scratch > > The 3 extents being created and subsequently deleted are: > > sync-799 [000] .... 313.938048: btrfs_update_block_group: Pinning 1943011328-2077229055 > sync-799 [000] .... 313.938073: btrfs_update_block_group: Pinning 2077229056-2177892351 <- BG1 ends > sync-799 [000] .... 313.938116: btrfs_update_block_group: Pinning 2177892352-2257584127 <- BG2 begins > > But we only get 1 discard request: > > sync-798 [003] .... 154.077897: btrfs_finish_extent_commit: Discarding 1943011328-2257584127 (len: 314572800) <- this is the request passed to btrfs_discard_extent > sync-798 [003] .... 154.077901: btrfs_discard_extent: Discarding 234881024 length for bytenr: 1943011328 <- this is the actual range being discarded inside the for loop. > > So the bug is genuine I will test whether your patch fixes it and report back. > >> Furthermore, this bug is not that reliably observed, as if the whole >> block group is empty, there will be another trim for that block group. > > Not only because of this, mainly because of the contiguousness requirement. Contiguousness requirement is in fact pretty easy to hit. Just do a 20G write, you will find most bg and file extents are Contiguous. The problem is to craft a reliable way extent and bg layout in a reproducible way, and a good way to detect the missing trim. In my environment, I'm using 20G write so even space cache is screwing me up, I still have 20 chances to have contiguous bg and file extents. And for trim result, loopback device can be used to account how many bytes are really used. I just need to polish them into a good fstests case. Thanks, Qu > >> >> So the most obvious way to find this missing trim needs to delete large >> extents at block group boundary without empting involved block groups. >> >> [FIX] >> - Allow __btrfs_map_block_for_discard() to modify @length parameter >> btrfs_map_block() uses its @length paramter to notify the caller how >> many bytes are mapped in current call. >> With __btrfs_map_block_for_discard() also modifing the @length, >> btrfs_discard_extent() now understands when to do extra trim. >> >> - Call btrfs_map_block() in a loop until we hit the range end >> Since we now know how many bytes are mapped each time, we can iterate >> through each block group boundary and issue correct trim for each >> range. >> >> Signed-off-by: Qu Wenruo <wqu@xxxxxxxx> > > <snip> >
