[cc willy and fsdevel] On Tue, Dec 19, 2023 at 07:21:37AM +1100, Dave Chinner wrote: > [cc Jan Kara] > > On Mon, Dec 18, 2023 at 07:29:34AM -0500, Brian Foster wrote: > > On Sat, Dec 16, 2023 at 09:39:51AM -0800, Darrick J. Wong wrote: > > > On Sat, Dec 16, 2023 at 07:55:59PM +0800, Long Li wrote: > > > > While fsstress + drop cache test, we get following warning: > > > > > > > > ------------[ cut here ]------------ > > > > WARNING: CPU: 2 PID: 1003 at fs/iomap/buffered-io.c:1182 iomap_file_buffered_write_punch_delalloc+0x691/0x730 > > > > Modules linked in: > > > > CPU: 2 PID: 1003 Comm: fsstress Not tainted 6.7.0-rc5-06945-g3ba9b31d6bf3-dirty #256 > > > > Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS ?-20190727_073836-buildvm-ppc64le-16.ppc.fedoraproject.org-3.fc31 04/01/2014 > > > > RIP: 0010:iomap_file_buffered_write_punch_delalloc+0x691/0x730 > > > > Code: d1 0b 01 0f 0b 48 83 05 14 a2 d1 0b 01 48 89 05 35 a1 d1 0b 49 39 ec 0f 8c 09 fb ff ff e9 b6 fd ff ff 48 83 05 df a1 d1 0b 01 <0f> 0b 48 83 05 dd a1 d1 0b 01 48 39 6c 24 10 7c c0 48 89 05 07 a1 > > > > RSP: 0018:ffffc900005b7b08 EFLAGS: 00010202 > > > > RAX: 0000000000000001 RBX: ffff888102363d40 RCX: 0000000000000001 > > > > RDX: 0000000000000000 RSI: 0000000000000000 RDI: ffff888108080000 > > > > RBP: 0000000000050000 R08: ffff888108084eb8 R09: ffff888108084eb8 > > > > R10: 000000000000005c R11: 0000000000000059 R12: 0000000000050000 > > > > R13: ffffffff8c978ef0 R14: 0000000000050000 R15: 000000000005a000 > > > > FS: 00007efc04c63b40(0000) GS:ffff88813bd00000(0000) knlGS:0000000000000000 > > > > CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 > > > > CR2: 00007efc0375c000 CR3: 0000000105a4d000 CR4: 00000000000006f0 > > > > DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000 > > > > DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400 > > > > Call Trace: > > > > <TASK> > > > > xfs_buffered_write_iomap_end+0x40/0xb0 > > > > iomap_iter+0x8e/0x5f0 > > > > iomap_file_buffered_write+0xa4/0x460 > > > > xfs_file_buffered_write+0x156/0x3d0 > > > > xfs_file_write_iter+0xb2/0x1c0 > > > > do_iter_readv_writev+0x19b/0x1f0 > > > > vfs_writev+0x114/0x4f0 > > > > do_writev+0x7f/0x1c0 > > > > __x64_sys_writev+0x24/0x30 > > > > do_syscall_64+0x3f/0xe0 > > > > entry_SYSCALL_64_after_hwframe+0x62/0x6a > > > > RIP: 0033:0x7efc03b06610 > > > > Code: 73 01 c3 48 8b 0d 78 88 2c 00 f7 d8 64 89 01 48 83 c8 ff c3 66 0f 1f 44 00 00 83 3d d9 e0 2c 00 00 75 10 b8 14 00 00 00 0f 05 <48> 3d 01 f0 ff ff 73 31 c3 48 83 ec 08 e8 5e 8e 01 00 48 89 04 24 > > > > RSP: 002b:00007ffdf8f426d8 EFLAGS: 00000246 ORIG_RAX: 0000000000000014 > > > > RAX: ffffffffffffffda RBX: 000000000000007a RCX: 00007efc03b06610 > > > > RDX: 00000000000002c4 RSI: 00000000012f5580 RDI: 0000000000000003 > > > > RBP: 0000000000000003 R08: 00000000012f53a0 R09: 0000000000000077 > > > > R10: 000000000000007c R11: 0000000000000246 R12: 00000000000002c4 > > > > R13: 00000000012dba50 R14: 00000000012f5580 R15: 0000000000000094 > > > > > > > > The warning occurred in the following code of iomap_write_delalloc_release(). > > > > After analyzing vmcore, I found that the reason for the warning is that > > > > data_end was equal to start_byte. > > > > > > > > WARN_ON_ONCE(data_end <= start_byte); > > > > > > > > If some delay is added between seeking for data and seeking for hole > > > > in iomap_write_delalloc_release(), the problem can be reproduced quickly. > > > > The root cause of the problem is that clean data page was dropped between > > > > two seeking in the page cache. As a result, data_end may be equal to > > > > start_byte. > > > > > > > > buffered write drop cache > > > > --------------------------- --------------------------- > > > > xfs_buffered_write_iomap_end > > > > iomap_file_buffered_write_punch_delalloc > > > > iomap_write_delalloc_release > > > > start_byte = mapping_seek_hole_data(SEEK_DATA) > > > > > > > > drop_pagecache_sb > > > > invalidate_mapping_pages > > > > mapping_try_invalidate > > > > mapping_evict_folio > > > > remove_mapping > > > > > > > > data_end = mapping_seek_hole_data(SEEK_HOLE) > > > > WARN_ON_ONCE(data_end <= start_byte) > > > > > > > > In my investigation, I found that clean data pages was alloced and added > > > > to page cache when reading the file's hole. After that, while buffered > > > > write and goes into delalloc release, we seek for data, it will find > > > > the start offset of the clean data pages. If the clean data pages was > > > > dropped, when we seek for hole, it will find the same offset as the > > > > previous seek. > > > > > > iomap_write_delalloc_release holds the invalidation lock, shouldn't that > > > be sufficient to prevent folios from being dropped? > > > > > > > I believe it's the other way around. The invalidate lock prevents new > > folios from being added (i.e. to serialize an invalidation sequence). > > IIRC it won't necessarily prevent folios from going away. > > It also serialises operations doing invalidations against other > operations doing invalidations (e.g. truncate, hole punch, direct > IO, etc) that require the invalidation lock to be held exclusively. > > Having looked a little deeper, it appears we have a large number of > invalidation paths that don't actually hold the invalidation lock at > all. drop_pagecache_sb() is just one of them. Is that a bug? Clearly I think so. ;) > If not, then this code needs to prevent any mapping tree change from > occurring whilst it is walking the range of the write that needs to > be scanned. The write has partially failed at this point, and we're > trying to clean up the mess the partial write has left behind. We > really need exclusive access to that range of the mapping whilst > cleanup is done. > > IOWs, if the invalidation lock is not sufficient to serialise > against other invalidations being run concurrently, what mapping > lock can we hold whilst we call into the filesystem, take sleeping > FS locks and modify the extent tree that will serialise against > other invalidations? I don't know of any other locks that would provide that; also, I thought invalidate_lock (fka MMAPLOCK_EXCL) was what xfs was supposed to take to serialize against folios getting ripped out of the mapping and page faults. Though now that I think about this further -- have dropcaches and xfs post-write error delalloc punching always lacked coordination? I think for most pagecache operations that old "lock and recheck mapping/offset" strategy works ok; it's just here where we might've been touching multiple folios that things go awry...? --D > -Dave. > > -- > Dave Chinner > david@xxxxxxxxxxxxx >
