https://bugzilla.kernel.org/show_bug.cgi?id=207053 --- Comment #6 from darrick.wong@xxxxxxxxxx --- On Tue, Apr 07, 2020 at 12:37:39PM -0400, Brian Foster wrote: > On Tue, Apr 07, 2020 at 08:17:38AM -0700, Darrick J. Wong wrote: > > On Tue, Apr 07, 2020 at 09:18:12AM -0400, Brian Foster wrote: > > > On Tue, Apr 07, 2020 at 06:41:31AM +0000, > bugzilla-daemon@xxxxxxxxxxxxxxxxxxx wrote: > > > > https://bugzilla.kernel.org/show_bug.cgi?id=207053 > > > > > > > > --- Comment #2 from Paul Furtado (paulfurtado91@xxxxxxxxx) --- > > > > Hi Dave, > > > > > > > > Just had another case of this crop up and I was able to get the blocked > tasks > > > > output before automation killed the server. Because the log was too > large to > > > > attach, I've pasted the output into a github gist here: > > > > > https://gist.githubusercontent.com/PaulFurtado/c9bade038b8a5c7ddb53a6e10def058f/raw/ee43926c96c0d6a9ec81a648754c1af599ef0bdd/sysrq_w.log > > > > > > > > > > Hm, so it looks like this is stuck between freeze: > > > > > > [377279.630957] fsfreeze D 0 46819 46337 0x00004084 > > > [377279.634910] Call Trace: > > > [377279.637594] ? __schedule+0x292/0x6f0 > > > [377279.640833] ? xfs_xattr_get+0x51/0x80 [xfs] > > > [377279.644287] schedule+0x2f/0xa0 > > > [377279.647286] schedule_timeout+0x1dd/0x300 > > > [377279.650661] wait_for_completion+0x126/0x190 > > > [377279.654154] ? wake_up_q+0x80/0x80 > > > [377279.657277] ? work_busy+0x80/0x80 > > > [377279.660375] __flush_work+0x177/0x1b0 > > > [377279.663604] ? worker_attach_to_pool+0x90/0x90 > > > [377279.667121] __cancel_work_timer+0x12b/0x1b0 > > > [377279.670571] ? rcu_sync_enter+0x8b/0xd0 > > > [377279.673864] xfs_stop_block_reaping+0x15/0x30 [xfs] > > > [377279.677585] xfs_fs_freeze+0x15/0x40 [xfs] > > > [377279.680950] freeze_super+0xc8/0x190 > > > [377279.684086] do_vfs_ioctl+0x510/0x630 > > > ... > > > > > > ... and the eofblocks scanner: > > > > > > [377279.422496] Workqueue: xfs-eofblocks/nvme13n1 xfs_eofblocks_worker > [xfs] > > > [377279.426971] Call Trace: > > > [377279.429662] ? __schedule+0x292/0x6f0 > > > [377279.432839] schedule+0x2f/0xa0 > > > [377279.435794] rwsem_down_read_slowpath+0x196/0x530 > > > [377279.439435] ? kmem_cache_alloc+0x152/0x1f0 > > > [377279.442834] ? __percpu_down_read+0x49/0x60 > > > [377279.446242] __percpu_down_read+0x49/0x60 > > > [377279.449586] __sb_start_write+0x5b/0x60 > > > [377279.452869] xfs_trans_alloc+0x152/0x160 [xfs] > > > [377279.456372] xfs_free_eofblocks+0x12d/0x1f0 [xfs] > > > [377279.460014] xfs_inode_free_eofblocks+0x128/0x1a0 [xfs] > > > [377279.463903] ? xfs_inode_ag_walk_grab+0x5f/0x90 [xfs] > > > [377279.467680] xfs_inode_ag_walk.isra.17+0x1a7/0x410 [xfs] > > > [377279.471567] ? __xfs_inode_clear_blocks_tag+0x120/0x120 [xfs] > > > [377279.475620] ? kvm_sched_clock_read+0xd/0x20 > > > [377279.479059] ? sched_clock+0x5/0x10 > > > [377279.482184] ? __xfs_inode_clear_blocks_tag+0x120/0x120 [xfs] > > > [377279.486234] ? radix_tree_gang_lookup_tag+0xa8/0x100 > > > [377279.489974] ? __xfs_inode_clear_blocks_tag+0x120/0x120 [xfs] > > > [377279.494041] xfs_inode_ag_iterator_tag+0x73/0xb0 [xfs] > > > [377279.497859] xfs_eofblocks_worker+0x29/0x40 [xfs] > > > [377279.501484] process_one_work+0x195/0x380 > > > ... > > > > > > The immediate issue is likely that the eofblocks transaction is not > > > NOWRITECOUNT (same for the cowblocks scanner, btw), but the problem with > > > doing that is these helpers are called from other contexts outside of > > > the background scanners. > > > > > > Perhaps what we need to do here is let these background scanners acquire > > > a superblock write reference, similar to what Darrick recently added to > > > scrub..? We'd have to do that from the scanner workqueue task, so it > > > would probably need to be a trylock so we don't end up in a similar > > > situation as above. I.e., we'd either get the reference and cause freeze > > > to wait until it's dropped or bail out if freeze has already stopped the > > > transaction subsystem. Thoughts? > > > > Hmm, I had a whole gigantic series to refactor all the speculative > > preallocation gc work into a single thread + radix tree tag; I'll see if > > that series actually fixed this problem too. > > > > But yes, all background threads that run transactions need to have > > freezer protection. > > > > So something like the following in the meantime, assuming we want a > backportable fix..? I think this means we could return -EAGAIN from the > eofblocks ioctl, but afaict if something functionally conflicts with an > active scan across freeze then perhaps that's preferred. Apparently I don't have a patch that fixes the speculative gc code. The deferred inactivation worker does it, so perhaps I got mixed up. :/ I think a better fix would be to annotate xfs_icache_free_eofblocks and xfs_icache_free_cowblocks to note that the caller must obtain freeze protection before calling those functions. Then we can play whackamole with the existing callers: 1. xfs_eofblocks_worker and xfs_cowblocks_worker can try to sb_start_write and just go back to sleep if the fs is frozen. The flush_workqueue will then cancel the delayed work and the freeze can proceed. 2. The buffered write ENOSPC scour-and-retry loops already have freeze protection because they're file writes, so they don't have to change. 3. XFS_IOC_FREE_EOFBLOCKS can sb_start_write, which means that callers will sleep on the frozen fs. --D > Brian > > diff --git a/fs/xfs/xfs_icache.c b/fs/xfs/xfs_icache.c > index a7be7a9e5c1a..0f14d58e5bb0 100644 > --- a/fs/xfs/xfs_icache.c > +++ b/fs/xfs/xfs_icache.c > @@ -1515,13 +1515,24 @@ __xfs_icache_free_eofblocks( > void *args), > int tag) > { > - int flags = SYNC_TRYLOCK; > + int flags = SYNC_TRYLOCK; > + int error; > > if (eofb && (eofb->eof_flags & XFS_EOF_FLAGS_SYNC)) > flags = SYNC_WAIT; > > - return xfs_inode_ag_iterator_tag(mp, execute, flags, > - eofb, tag); > + /* > + * freeze waits on background scanner jobs to complete so we cannot > + * block on write protection here. Bail if the transaction subsystem is > + * already freezing, returning -EAGAIN to notify other callers. > + */ > + if (!sb_start_write_trylock(mp->m_super)) > + return -EAGAIN; > + > + error = xfs_inode_ag_iterator_tag(mp, execute, flags, eofb, tag); > + sb_end_write(mp->m_super); > + > + return error; > } > > int > > > --D > > > > > Brian > > > > > > > > > > > Thanks, > > > > Paul > > > > > > > > -- > > > > You are receiving this mail because: > > > > You are watching the assignee of the bug. > > > > > > > > > > -- You are receiving this mail because: You are watching the assignee of the bug.
