[PATCH v3 01/11] fs: fix lazytime expiration handling in __writeback_single_inode()

Eric Biggers
2021-01-12T19:05:07Z

When lazytime is enabled and an inode is being written due to its
in-memory updated timestamps having expired, either due to a sync() or
syncfs() system call or due to dirtytime_expire_interval having elapsed,
the VFS needs to inform the filesystem so that the filesystem can copy
the inode's timestamps out to the on-disk data structures.

This is done by __writeback_single_inode() calling
mark_inode_dirty_sync(), which then calls ->dirty_inode(I_DIRTY_SYNC).

However, this occurs after __writeback_single_inode() has already
cleared the dirty flags from ->i_state. This causes two bugs:

- mark_inode_dirty_sync() redirties the inode, causing it to remain
dirty. This wastefully causes the inode to be written twice. But
more importantly, it breaks cases where sync_filesystem() is expected
to clean dirty inodes. This includes the FS_IOC_REMOVE_ENCRYPTION_KEY
ioctl (as reported at
https://lore.kernel.org/r/20200306004555.GB225345@gmail.com), as well
as possibly filesystem freezing (freeze_super()).

- Since ->i_state doesn't contain I_DIRTY_TIME when ->dirty_inode() is
called from __writeback_single_inode() for lazytime expiration,
xfs_fs_dirty_inode() ignores the notification. (XFS only cares about
lazytime expirations, and it assumes that i_state will contain
I_DIRTY_TIME during those.) Therefore, lazy timestamps aren't
persisted by sync(), syncfs(), or dirtytime_expire_interval on XFS.

Fix this by moving the call to mark_inode_dirty_sync() to earlier in
__writeback_single_inode(), before the dirty flags are cleared from
i_state. This makes filesystems be properly notified of the timestamp
expiration, and it avoids incorrectly redirtying the inode.

This fixes xfstest generic/580 (which tests
FS_IOC_REMOVE_ENCRYPTION_KEY) when run on ext4 or f2fs with lazytime
enabled. It also fixes the new lazytime xfstest I've proposed, which
reproduces the above-mentioned XFS bug
(https://lore.kernel.org/r/20210105005818.92978-1-ebiggers@kernel.org).

Alternatively, we could call ->dirty_inode(I_DIRTY_SYNC) directly. But
due to the introduction of I_SYNC_QUEUED, mark_inode_dirty_sync() is the
right thing to do because mark_inode_dirty_sync() now knows not to move
the inode to a writeback list if it is currently queued for sync.

Fixes: 0ae45f63d4ef ("vfs: add support for a lazytime mount option")
Cc: stable@vger.kernel.org
Depends-on: 5afced3bf281 ("writeback: Avoid skipping inode writeback")
Suggested-by: Jan Kara <jack@suse.cz>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Eric Biggers <ebiggers@google.com>
---
fs/fs-writeback.c | 24 +++++++++++++-----------
1 file changed, 13 insertions(+), 11 deletions(-)

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index acfb55834af23..c41cb887eb7d3 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -1474,21 +1474,25 @@ __writeback_single_inode(struct inode *inode, struct writeback_control *wbc)
}

/*
- * Some filesystems may redirty the inode during the writeback
- * due to delalloc, clear dirty metadata flags right before
- * write_inode()
+ * If the inode has dirty timestamps and we need to write them, call
+ * mark_inode_dirty_sync() to notify the filesystem about it and to
+ * change I_DIRTY_TIME into I_DIRTY_SYNC.
*/
- spin_lock(&inode->i_lock);
-
- dirty = inode->i_state & I_DIRTY;
if ((inode->i_state & I_DIRTY_TIME) &&
- ((dirty & I_DIRTY_INODE) ||
- wbc->sync_mode == WB_SYNC_ALL || wbc->for_sync ||
+ (wbc->sync_mode == WB_SYNC_ALL || wbc->for_sync ||
time_after(jiffies, inode->dirtied_time_when +
dirtytime_expire_interval * HZ))) {
- dirty |= I_DIRTY_TIME;
trace_writeback_lazytime(inode);
+ mark_inode_dirty_sync(inode);
}
+
+ /*
+ * Some filesystems may redirty the inode during the writeback
+ * due to delalloc, clear dirty metadata flags right before
+ * write_inode()
+ */
+ spin_lock(&inode->i_lock);
+ dirty = inode->i_state & I_DIRTY;
inode->i_state &= ~dirty;

/*
@@ -1509,8 +1513,6 @@ __writeback_single_inode(struct inode *inode, struct writeback_control *wbc)

spin_unlock(&inode->i_lock);

- if (dirty & I_DIRTY_TIME)
- mark_inode_dirty_sync(inode);
/* Don't write the inode if only I_DIRTY_PAGES was set */
if (dirty & ~I_DIRTY_PAGES) {
int err = write_inode(inode, wbc);
--
2.30.0