Re: [Kernel Bug] WARNING in mempool_alloc_noprof

[Eric Biggers <ebiggers@kernel.org>]

On Tue, Feb 03, 2026 at 07:30:07PM +0100, Vlastimil Babka wrote:
> On 2/3/26 17:59, Christoph Hellwig wrote:
> > On Tue, Feb 03, 2026 at 05:55:27PM +0100, Vlastimil Babka wrote:
> >> On 2/3/26 17:27, Christoph Hellwig wrote:
> >> > On Tue, Feb 03, 2026 at 06:52:39PM +0900, Harry Yoo wrote:
> >> >> Maybe the changelog could be rephrased a bit,
> >> >> but overall LGTM, thanks!
> >> > 
> >> > 
> >> > No, that does not make sense.  If mempool is used with __GFP_RECLAIM in
> >> > the flags it won't fail, and if it isn't, GFP_NOFAIL can't work.
> >> 
> >> So that means as long as there's __GFP_RECLAIM, __GFP_NOFAIL isn't wrong,
> >> just redundant.
> > 
> > Given how picky the rest of the mm is about __GFP_NOFAIL, silently
> > accepting it where it has no (or a weird and unexpected) effect
> > seems like a disservice to the users.
> 
> OK then. But I don't think we need to add checks to the mempool hot paths.
> If somebody uses __GFP_NOFAIL, eventually it will trickle to the existing
> warning that triggered here. If it's using slab then eventually that will
> reach the page allocator too. Maybe not with some custom alloc functions,
> but meh.
> 
> This f2fs_encrypt_one_page() case is weird though (and the relevant parts
> seem to be identical in current mainline).
> It uses GFP_NOFS, so __GFP_RECLAIM is there. It only adds __GFP_NOFAIL in
> case fscrypt_encrypt_pagecache_blocks() already failed with -ENOMEM.
> 
> That means fscrypt_alloc_bounce_page() returns NULL, which is either the
> WARN_ON_ONCE(!fscrypt_bounce_page_pool) case (but the report doesn't include
> such a warning), or mempool_alloc() failed - but that shouldn't happen with
> GFP_NOFS?
> 
> (Also the !fscrypt_bounce_page_pool is therefore an infinite retry loop,
> isn't it? Which would be truly a bug, unless I'm missing something.)
> 
> Ah but fscrypt_encrypt_pagecache_blocks() can also return -ENOMEM due to
> fscrypt_crypt_data_unit() returning it.
> 
> And there theoretically in v6.12.11 skcipher_request_alloc() could return
> -ENOMEM. In practice I assume this report was achieved by fault injection.
> But that possibility is gone with mainline commit 52e7e0d88933 ("fscrypt:
> Switch to sync_skcipher and on-stack requests") anyway.
> 
> I think the whole "goto retry_encrypt;" loop in f2fs_encrypt_one_page()
> should just be removed.

Indeed, fscrypt_encrypt_pagecache_blocks() (or the older code it was
derived from) used to do multiple memory allocations.  Now it only
allocates the bounce page itself.

Also, the intended usage is what ext4 does: use GFP_NOFS for the first
page in the bio for a guaranteed allocation from the mempool, then
GFP_NOWAIT for any subsequent pages.  If any of the subsequent
allocations fails, ext4 submits the bio early and starts a new one.

f2fs does it differently and just always uses GFP_NOFS.  Yes, that
doesn't make sense.  I guess ideally it would be changed to properly do
opportunistic allocations in the same way as ext4.  But until then, just
removing the retry loop sounds good.

- Eric