6.15-stable review patch. If anyone has any objections, please let me know.
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From: Alistair Popple apopple@nvidia.com
[ Upstream commit dd59137bfe70cf3646021b4721e430213b9c71bd ]
Commit 6be3e21d25ca ("fs/dax: don't skip locked entries when scanning entries") introduced a new function, wait_entry_unlocked_exclusive(), which waits for the current entry to become unlocked without advancing the XArray iterator state.
Waiting for the entry to become unlocked requires dropping the XArray lock. This requires calling xas_pause() prior to dropping the lock which leaves the xas in a suitable state for the next iteration. However this has the side-effect of advancing the xas state to the next index. Normally this isn't an issue because xas_for_each() contains code to detect this state and thus avoid advancing the index a second time on the next loop iteration.
However both callers of and wait_entry_unlocked_exclusive() itself subsequently use the xas state to reload the entry. As xas_pause() updated the state to the next index this will cause the current entry which is being waited on to be skipped. This caused the following warning to fire intermittently when running xftest generic/068 on an XFS filesystem with FS DAX enabled:
[ 35.067397] ------------[ cut here ]------------ [ 35.068229] WARNING: CPU: 21 PID: 1640 at mm/truncate.c:89 truncate_folio_batch_exceptionals+0xd8/0x1e0 [ 35.069717] Modules linked in: nd_pmem dax_pmem nd_btt nd_e820 libnvdimm [ 35.071006] CPU: 21 UID: 0 PID: 1640 Comm: fstest Not tainted 6.15.0-rc7+ #77 PREEMPT(voluntary) [ 35.072613] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS rel-1.16.3-0-ga6ed6b701f0a-prebuilt.qemu.org 04/01/204 [ 35.074845] RIP: 0010:truncate_folio_batch_exceptionals+0xd8/0x1e0 [ 35.075962] Code: a1 00 00 00 f6 47 0d 20 0f 84 97 00 00 00 4c 63 e8 41 39 c4 7f 0b eb 61 49 83 c5 01 45 39 ec 7e 58 42 f68 [ 35.079522] RSP: 0018:ffffb04e426c7850 EFLAGS: 00010202 [ 35.080359] RAX: 0000000000000000 RBX: ffff9d21e3481908 RCX: ffffb04e426c77f4 [ 35.081477] RDX: ffffb04e426c79e8 RSI: ffffb04e426c79e0 RDI: ffff9d21e34816e8 [ 35.082590] RBP: ffffb04e426c79e0 R08: 0000000000000001 R09: 0000000000000003 [ 35.083733] R10: 0000000000000000 R11: 822b53c0f7a49868 R12: 000000000000001f [ 35.084850] R13: 0000000000000000 R14: ffffb04e426c78e8 R15: fffffffffffffffe [ 35.085953] FS: 00007f9134c87740(0000) GS:ffff9d22abba0000(0000) knlGS:0000000000000000 [ 35.087346] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 [ 35.088244] CR2: 00007f9134c86000 CR3: 000000040afff000 CR4: 00000000000006f0 [ 35.089354] Call Trace: [ 35.089749] <TASK> [ 35.090168] truncate_inode_pages_range+0xfc/0x4d0 [ 35.091078] truncate_pagecache+0x47/0x60 [ 35.091735] xfs_setattr_size+0xc7/0x3e0 [ 35.092648] xfs_vn_setattr+0x1ea/0x270 [ 35.093437] notify_change+0x1f4/0x510 [ 35.094219] ? do_truncate+0x97/0xe0 [ 35.094879] do_truncate+0x97/0xe0 [ 35.095640] path_openat+0xabd/0xca0 [ 35.096278] do_filp_open+0xd7/0x190 [ 35.096860] do_sys_openat2+0x8a/0xe0 [ 35.097459] __x64_sys_openat+0x6d/0xa0 [ 35.098076] do_syscall_64+0xbb/0x1d0 [ 35.098647] entry_SYSCALL_64_after_hwframe+0x77/0x7f [ 35.099444] RIP: 0033:0x7f9134d81fc1 [ 35.100033] Code: 75 57 89 f0 25 00 00 41 00 3d 00 00 41 00 74 49 80 3d 2a 26 0e 00 00 74 6d 89 da 48 89 ee bf 9c ff ff ff5 [ 35.102993] RSP: 002b:00007ffcd41e0d10 EFLAGS: 00000202 ORIG_RAX: 0000000000000101 [ 35.104263] RAX: ffffffffffffffda RBX: 0000000000000242 RCX: 00007f9134d81fc1 [ 35.105452] RDX: 0000000000000242 RSI: 00007ffcd41e1200 RDI: 00000000ffffff9c [ 35.106663] RBP: 00007ffcd41e1200 R08: 0000000000000000 R09: 0000000000000064 [ 35.107923] R10: 00000000000001a4 R11: 0000000000000202 R12: 0000000000000066 [ 35.109112] R13: 0000000000100000 R14: 0000000000100000 R15: 0000000000000400 [ 35.110357] </TASK> [ 35.110769] irq event stamp: 8415587 [ 35.111486] hardirqs last enabled at (8415599): [<ffffffff8d74b562>] __up_console_sem+0x52/0x60 [ 35.113067] hardirqs last disabled at (8415610): [<ffffffff8d74b547>] __up_console_sem+0x37/0x60 [ 35.114575] softirqs last enabled at (8415300): [<ffffffff8d6ac625>] handle_softirqs+0x315/0x3f0 [ 35.115933] softirqs last disabled at (8415291): [<ffffffff8d6ac811>] __irq_exit_rcu+0xa1/0xc0 [ 35.117316] ---[ end trace 0000000000000000 ]---
Fix this by using xas_reset() instead, which is equivalent in implementation to xas_pause() but does not advance the XArray state.
Fixes: 6be3e21d25ca ("fs/dax: don't skip locked entries when scanning entries") Signed-off-by: Alistair Popple apopple@nvidia.com Link: https://lore.kernel.org/20250523043749.1460780-1-apopple@nvidia.com Reviewed-by: Dan Williams dan.j.williams@intel.com Reviewed-by: Jan Kara jack@suse.cz Cc: Dan Williams dan.j.williams@intel.com Cc: Alison Schofield alison.schofield@intel.com Cc: "Matthew Wilcow (Oracle)" willy@infradead.org Cc: Balbir Singh balbirs@nvidia.com Cc: "Darrick J. Wong" djwong@kernel.org Cc: Dave Chinner david@fromorbit.com Cc: David Hildenbrand david@redhat.com Cc: Jan Kara jack@suse.cz Cc: John Hubbard jhubbard@nvidia.com Cc: Ted Ts'o tytso@mit.edu Cc: Alexander Viro viro@zeniv.linux.org.uk Cc: Christian Brauner brauner@kernel.org Signed-off-by: Christian Brauner brauner@kernel.org Signed-off-by: Sasha Levin sashal@kernel.org --- fs/dax.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/fs/dax.c b/fs/dax.c index 676303419e9e8..f8d8b1afd2324 100644 --- a/fs/dax.c +++ b/fs/dax.c @@ -257,7 +257,7 @@ static void *wait_entry_unlocked_exclusive(struct xa_state *xas, void *entry) wq = dax_entry_waitqueue(xas, entry, &ewait.key); prepare_to_wait_exclusive(wq, &ewait.wait, TASK_UNINTERRUPTIBLE); - xas_pause(xas); + xas_reset(xas); xas_unlock_irq(xas); schedule(); finish_wait(wq, &ewait.wait);