Re: [PATCH] ext4: avoid __GFP_NOFAIL in __ext4_get_inode_loc allocation

["Theodore Tso" <tytso@mit.edu>]

On Mon, Apr 27, 2026 at 06:23:00PM -0400, Chao Shi wrote:
> When kswapd shrinks the dcache, the last iput() on an ext4 inode can
> trigger ext4_orphan_del(), which calls ext4_reserve_inode_write() and
> ultimately __ext4_get_inode_loc().  That function calls sb_getblk(),
> which wraps __getblk() and carries implicit __GFP_NOFAIL.  Because
> kswapd runs with PF_MEMALLOC set, combining NOFAIL with a non-reclaimable
> context trips WARN_ON_ONCE(current->flags & PF_MEMALLOC) inside
> __alloc_pages_slowpath(), producing a spurious splat even though the
> allocation could simply fail and return -ENOMEM to the caller.

NAK.  As Sashiko correctly points out:

Sashiko AI review found 1 potential issue(s):
- [Critical] Removing __GFP_NOFAIL from __ext4_get_inode_loc causes transient memory
shortages to trigger a fatal filesystem abort (remount read-only) or severe metadata
corruption, trading a memory reclaim warning for a Denial of Service.

The warning in mm/page_alloc.c is the sort of thing that causes file
system developers to decide to drop __GFP_NOFAIL and replace it with a
retry loop just to shut the mm subsystem the heck up, since some mm
developers seem to view hangs in heavy OOM conditions as the worst
thing, where as fs developers consider data corruption to be far
worse, since users tend to get cranky when they lose their data, and
(a) in practice the OOM killer tends to get triggered first, and (b)
that's what software and hardware watchdogs are for.

In any case, there are *far* worse things than a random splat, and if
you really want to make it go away, my suggestion is to remove the
WARN_ON_ONCE from __alloc_pages_slowpath().

		/*
		 * PF_MEMALLOC request from this context is rather bizarre
		 * because we cannot reclaim anything and only can loop waiting
		 * for somebody to do a work for us.
		 */
		WARN_ON_ONCE(current->flags & PF_MEMALLOC);

I disagrr the premise; it's not bizzare at all.

						- Ted