On Thu, Sep 18, 2025 at 02:04:52AM +0200, Mateusz Guzik wrote:
On Thu, Sep 18, 2025 at 1:08 AM Mateusz Guzik mjguzik@gmail.com wrote:
On Thu, Sep 18, 2025 at 12:51 AM Dave Chinner david@fromorbit.com wrote:
- wait for Josef to finish his inode refcount rework patchset that gets rid of this whole "writeback doesn't hold an inode reference" problem that is the root cause of this the deadlock.
All that adding a whacky async iput work around does right now is make it harder for Josef to land the patchset that makes this problem go away entirely....
Per Max this is a problem present on older kernels as well, something of this sort is needed to cover it regardless of what happens in mainline.
As for mainline, I don't believe Josef's patchset addresses the problem.
The newly added refcount now taken by writeback et al only gates the inode getting freed, it does not gate almost any of iput/evict processing. As in with the patchset writeback does not hold a real reference.
So ceph can still iput from writeback and find itself waiting in inode_wait_for_writeback, unless the filesystem can be converted to use the weaker refcounts and iobj_put instead (but that's not something I would be betting on).
To further elaborate, an extra count which only gates the struct being freed has limited usefulness. Notably it does not help filesystems which need the inode to be valid for use the entire time as evict() is only stalled *after* ->evict_inode(), which might have destroyed the vital parts.
Not sure I follow you - ->evict_inode() comes after waiting for writeback and other VFS operations to complete. There's nothing in the VFS eviction code that actually blocks after ->evict_inode() is called.
Or to put it differently, the patchset tries to fit btrfs's needs which don't necessarily line up with other filesystems.
Yes, that much is obvious, I think it will be difficult to use it to replace any of XFS's custom asynchronous inode cleanup code at this point in time...
For example it may be ceph needs the full reference in writeback,
IMO, we should always hold a full reference in writeback, because doing so addresses the underlying eviction vs writeback race condition that exists. i.e. we currently handle the lack of reference counts in writeback by blocking on I_SYNC in eviction to prevent a UAF.
If we have an active reference for writeback in the first place then eviction doesn't need to block on writeback because, by definition, we cannot be performing writeback and eviction at the same time....
then the new ref is of no use here. But for the sake of argument let's say ceph will get away with the ligher ref instead. Even then this is on the clock for a different filesystem to show up which can't do it and needs an async iput and then its developers are looking at "whacky work arounds".
Right, that's because we haven't addressed the underlying problem.
That is, we need to hold the right references at the VFS level such that filesystems can't drop the last reference to the inode whilst high level VFS inode operations (such as writeback) are in progress on that inode.
Done properly, eviction can then be done asynchronously for all inodes because we've guaranteed there are no active or pending active users of the inode....
.... and at that point, all the custom async inode garbage collection stuff that XFS does goes away because it is native VFS functionality :)
The actual generic async iput is the actual async iput, not an arbitrary chunk of it after the inode is partway through processing. But then any form of extra refcounting is of no significance.
To that end a non-whacky mechanism to defer iput would be most welcome, presumably provided by the vfs layer itself. Per remarks by Al elsewhere, care needs to be taken to make sure all inodes are sorted out before the super block gets destroyed.
Yes, but first we have to get the reference counting right, such that inode eviction only occurs after we've guaranteed there are no other active users of the inode. Page cache residency and dirty inodes are still in active use, we should account for them that way.
This suggests expanding the super_block to track all of the deferred iputs and drain them early in sb destruction. The current struct inode on LP64 has 2 * 4 byte holes and llist linkage is only 8 bytes, so this can be added without growing the struct above stock kernel.
Right, we already do this lockless llist based async garbage collection under ->destroy_inode with XFS.
I would argue it would be good if the work could be deffered to task_work if possible (fput-style). Waiting for these should be easy enough, but arguably the thread which is supposed to get to them can be stalled elsewhere indefinitely, so perhaps this bit is a no-go.
If the reference counting is right, nothing expect a new lookup on the inode should stall on an inode queued for eviction...
-Dave.