On Wed, Jun 17, 2020 at 7:39 PM Boris Burkov <bo...@bur.io> wrote: > > Under somewhat convoluted conditions, it is possible to attempt to > release an extent_buffer that is under io, which triggers a BUG_ON in > btrfs_release_extent_buffer_pages. > > This relies on a few different factors. First, extent_buffer reads done > as readahead for searching use WAIT_NONE, so they free the local extent > buffer reference while the io is outstanding. However, they should still > be protected by TREE_REF. However, if the system is doing signficant > reclaim, and simultaneously heavily accessing the extent_buffers, it is > possible for releasepage to race with two concurrent readahead attempts > in a way that leaves TREE_REF unset when the readahead extent buffer is > released. > > Essentially, if two tasks race to allocate a new extent_buffer, but the > winner who attempts the first io is rebuffed by a page being locked > (likely by the reclaim itself) then the loser will still go ahead with > issuing the readahead. The loser's call to find_extent_buffer must also > race with the reclaim task reading the extent_buffer's refcount as 1 in > a way that allows the reclaim to re-clear the TREE_REF checked by > find_extent_buffer. > > The following represents an example execution demonstrating the race: > > CPU0 CPU1 > CPU2 > reada_for_search reada_for_search > readahead_tree_block > readahead_tree_block > find_create_tree_block > find_create_tree_block > alloc_extent_buffer > alloc_extent_buffer > > find_extent_buffer // not found > allocates eb > lock pages > associate > pages to eb > insert eb > into radix tree > set > TREE_REF, refs == 2 > unlock pages > > read_extent_buffer_pages // WAIT_NONE > not uptodate > (brand new eb) > > lock_page > if > !trylock_page > goto > unlock_exit // not an error > > free_extent_buffer > > release_extent_buffer > > atomic_dec_and_test refs to 1 > find_extent_buffer // found > > try_release_extent_buffer > > take refs_lock > > reads refs == 1; no io > atomic_inc_not_zero refs to 2 > mark_buffer_accessed > check_buffer_tree_ref > // not STALE, won't take refs_lock > refs == 2; TREE_REF set // no action > read_extent_buffer_pages // WAIT_NONE > > clear TREE_REF > > release_extent_buffer > > atomic_dec_and_test refs to 1 > > unlock_page > still not uptodate (CPU1 read failed on trylock_page) > locks pages > set io_pages > 0 > submit io > return > free_extent_buffer > release_extent_buffer > dec refs to 0 > delete from radix tree > btrfs_release_extent_buffer_pages > BUG_ON(io_pages > 0)!!! > > We observe this at a very low rate in production and were also able to > reproduce it in a test environment by introducing some spurious delays > and by introducing probabilistic trylock_page failures. > > To fix it, we apply check_tree_ref at a point where it could not > possibly be unset by a competing task: after io_pages has been > incremented. All the codepaths that clear TREE_REF check for io, so they > would not be able to clear it after this point until the io is done. > > stack trace, for reference: > [1417839.424739] ------------[ cut here ]------------ > [1417839.435328] kernel BUG at fs/btrfs/extent_io.c:4841! > [1417839.447024] invalid opcode: 0000 [#1] SMP > [1417839.502972] RIP: 0010:btrfs_release_extent_buffer_pages+0x20/0x1f0 > [1417839.517008] Code: ed e9 ... > [1417839.558895] RSP: 0018:ffffc90020bcf798 EFLAGS: 00010202 > [1417839.570816] RAX: 0000000000000002 RBX: ffff888102d6def0 RCX: > 0000000000000028 > [1417839.586962] RDX: 0000000000000002 RSI: ffff8887f0296482 RDI: > ffff888102d6def0 > [1417839.603108] RBP: ffff88885664a000 R08: 0000000000000046 R09: > 0000000000000238 > [1417839.619255] R10: 0000000000000028 R11: ffff88885664af68 R12: > 0000000000000000 > [1417839.635402] R13: 0000000000000000 R14: ffff88875f573ad0 R15: > ffff888797aafd90 > [1417839.651549] FS: 00007f5a844fa700(0000) GS:ffff88885f680000(0000) > knlGS:0000000000000000 > [1417839.669810] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 > [1417839.682887] CR2: 00007f7884541fe0 CR3: 000000049f609002 CR4: > 00000000003606e0 > [1417839.699037] DR0: 0000000000000000 DR1: 0000000000000000 DR2: > 0000000000000000 > [1417839.715187] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: > 0000000000000400 > [1417839.731320] Call Trace: > [1417839.737103] release_extent_buffer+0x39/0x90 > [1417839.746913] read_block_for_search.isra.38+0x2a3/0x370 > [1417839.758645] btrfs_search_slot+0x260/0x9b0 > [1417839.768054] btrfs_lookup_file_extent+0x4a/0x70 > [1417839.778427] btrfs_get_extent+0x15f/0x830 > [1417839.787665] ? submit_extent_page+0xc4/0x1c0 > [1417839.797474] ? __do_readpage+0x299/0x7a0 > [1417839.806515] __do_readpage+0x33b/0x7a0 > [1417839.815171] ? btrfs_releasepage+0x70/0x70 > [1417839.824597] extent_readpages+0x28f/0x400 > [1417839.833836] read_pages+0x6a/0x1c0 > [1417839.841729] ? startup_64+0x2/0x30 > [1417839.849624] __do_page_cache_readahead+0x13c/0x1a0 > [1417839.860590] filemap_fault+0x6c7/0x990 > [1417839.869252] ? xas_load+0x8/0x80 > [1417839.876756] ? xas_find+0x150/0x190 > [1417839.884839] ? filemap_map_pages+0x295/0x3b0 > [1417839.894652] __do_fault+0x32/0x110 > [1417839.902540] __handle_mm_fault+0xacd/0x1000 > [1417839.912156] handle_mm_fault+0xaa/0x1c0 > [1417839.921004] __do_page_fault+0x242/0x4b0 > [1417839.930044] ? page_fault+0x8/0x30 > [1417839.937933] page_fault+0x1e/0x30 > [1417839.945631] RIP: 0033:0x33c4bae > [1417839.952927] Code: Bad RIP value. > [1417839.960411] RSP: 002b:00007f5a844f7350 EFLAGS: 00010206 > [1417839.972331] RAX: 000000000000006e RBX: 1614b3ff6a50398a RCX: > 0000000000000000 > [1417839.988477] RDX: 0000000000000000 RSI: 0000000000000000 RDI: > 0000000000000002 > [1417840.004626] RBP: 00007f5a844f7420 R08: 000000000000006e R09: > 00007f5a94aeccb8 > [1417840.020784] R10: 00007f5a844f7350 R11: 0000000000000000 R12: > 00007f5a94aecc79 > [1417840.036932] R13: 00007f5a94aecc78 R14: 00007f5a94aecc90 R15: > 00007f5a94aecc40 > > Signed-off-by: Boris Burkov <bo...@bur.io>
Reviewed-by: Filipe Manana <fdman...@suse.com> Looks good, thanks! > --- > fs/btrfs/extent_io.c | 40 ++++++++++++++++++++++++---------------- > 1 file changed, 24 insertions(+), 16 deletions(-) > > diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c > index c59e07360083..95313bb7fe40 100644 > --- a/fs/btrfs/extent_io.c > +++ b/fs/btrfs/extent_io.c > @@ -5086,25 +5086,28 @@ struct extent_buffer > *alloc_dummy_extent_buffer(struct btrfs_fs_info *fs_info, > static void check_buffer_tree_ref(struct extent_buffer *eb) > { > int refs; > - /* the ref bit is tricky. We have to make sure it is set > - * if we have the buffer dirty. Otherwise the > - * code to free a buffer can end up dropping a dirty > - * page > + /* > + * The TREE_REF bit is first set when the extent_buffer is added > + * to the radix tree. It is also reset, if unset, when a new reference > + * is created by find_extent_buffer. > * > - * Once the ref bit is set, it won't go away while the > - * buffer is dirty or in writeback, and it also won't > - * go away while we have the reference count on the > - * eb bumped. > + * It is only cleared in two cases: freeing the last non-tree > + * reference to the extent_buffer when its STALE bit is set or > + * calling releasepage when the tree reference is the only reference. > * > - * We can't just set the ref bit without bumping the > - * ref on the eb because free_extent_buffer might > - * see the ref bit and try to clear it. If this happens > - * free_extent_buffer might end up dropping our original > - * ref by mistake and freeing the page before we are able > - * to add one more ref. > + * In both cases, care is taken to ensure that the extent_buffer's > + * pages are not under io. However, releasepage can be concurrently > + * called with creating new references, which is prone to race > + * conditions between the calls to check_buffer_tree_ref in those > + * codepaths and clearing TREE_REF in try_release_extent_buffer. > * > - * So bump the ref count first, then set the bit. If someone > - * beat us to it, drop the ref we added. > + * The actual lifetime of the extent_buffer in the radix tree is > + * adequately protected by the refcount, but the TREE_REF bit and > + * its corresponding reference are not. To protect against this > + * class of races, we call check_buffer_tree_ref from the codepaths > + * which trigger io after they set eb->io_pages. Note that once io is > + * initiated, TREE_REF can no longer be cleared, so that is the > + * moment at which any such race is best fixed. > */ > refs = atomic_read(&eb->refs); > if (refs >= 2 && test_bit(EXTENT_BUFFER_TREE_REF, &eb->bflags)) > @@ -5555,6 +5558,11 @@ int read_extent_buffer_pages(struct extent_buffer *eb, > int wait, int mirror_num) > clear_bit(EXTENT_BUFFER_READ_ERR, &eb->bflags); > eb->read_mirror = 0; > atomic_set(&eb->io_pages, num_reads); > + /* > + * It is possible for releasepage to clear the TREE_REF bit before we > + * set io_pages. See check_buffer_tree_ref for a more detailed > comment. > + */ > + check_buffer_tree_ref(eb); > for (i = 0; i < num_pages; i++) { > page = eb->pages[i]; > > -- > 2.24.1 > -- Filipe David Manana, “Whether you think you can, or you think you can't — you're right.”