On Thu, Aug 28, 2025 at 08:01:39AM +1000, Dave Chinner wrote:

On Wed, Aug 27, 2025 at 02:32:49PM +0200, Christian Brauner wrote:

quoted

On Tue, Aug 26, 2025 at 11:39:17AM -0400, Josef Bacik wrote:

quoted

We can end up with an inode on the LRU list or the cached list, then at
some point in the future go to unlink that inode and then still have an
elevated i_count reference for that inode because it is on one of these
lists.

The more common case is the cached list. We open a file, write to it,
truncate some of it which triggers the inode_add_lru code in the
pagecache, adding it to the cached LRU.  Then we unlink this inode, and
it exists until writeback or reclaim kicks in and removes the inode.

To handle this case, delete the inode from the LRU list when it is
unlinked, so we have the best case scenario for immediately freeing the
inode.

Signed-off-by: Josef Bacik <josef@toxicpanda.com>
---

I'm not too fond of this particular change I think it's really misplaced
and the correct place is indeed drop_nlink() and clear_nlink().

I don't really like putting it in drop_nlink because that then puts
the inode LRU in the middle of filesystem transactions when lots of
different filesystem locks are held.

IF the LRU operations are in the VFS, then we know exactly what
locks are held when it is performed (current behaviour). However,
when done from the filesystem transaction context running
drop_nlink, we'll have different sets of locks and/or execution
contexts held for each different fs type.

quoted

I'm pretty sure that the number of callers that hold i_lock around
drop_nlink() and clear_nlink() is relatively small.

I think the calling context problem is wider than the obvious issue
with i_lock....

This is an internal LRU, so yes potentially we could have locking issues, but
right now all LRU operations are nested inside of the i_lock, and this is purely
about object lifetime. I'm not concerned about this being in the bowls of any
filesystem because it's purely list manipulation.

And if it makes you feel better, the next patchset queued up for after the next
merge window is deleting the LRU, so you won't have to worry about it for long
:).  Thanks,

Josef