linux-ext4 - Re: [PATCH 1/3] quota: Prevent memory allocation recursion while holding dq

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Date:   Mon, 6 Jun 2022 15:08:09 +0200
From:   Jan Kara <jack@...e.cz>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Jan Kara <jack@...e.cz>, Jan Kara <jack@...e.com>, tytso@....edu,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH 1/3] quota: Prevent memory allocation recursion while
 holding dq_lock

On Mon 06-06-22 13:42:10, Matthew Wilcox wrote:
> On Mon, Jun 06, 2022 at 10:03:34AM +0200, Jan Kara wrote:
> > On Sun 05-06-22 15:38:13, Matthew Wilcox (Oracle) wrote:
> > > As described in commit 02117b8ae9c0 ("f2fs: Set GF_NOFS in
> > > read_cache_page_gfp while doing f2fs_quota_read"), we must not enter
> > > filesystem reclaim while holding the dq_lock.  Prevent this more generally
> > > by using memalloc_nofs_save() while holding the lock.
> > > 
> > > Signed-off-by: Matthew Wilcox (Oracle) <willy@...radead.org>
> > 
> > This is definitely a good cleanup to have and seems mostly unrelated to the
> > rest. I'll take it rightaway into my tree. Thanks for the patch!
> 
> Thanks!  It's really a pre-requisite for the second patch; I haven't
> seen anywhere in the current codebase that will have a problem.  All
> the buffer_heads are allocated with GFP_NOFS | __GFP_NOFAIL (in
> grow_dev_page()).

Yes, I understand. But as f2fs case shows, there can be fs-local
allocations that may be impacted. And it is good to have it documented in
the code that dq_lock is not reclaim safe to avoid bugs like f2fs had in
the future.

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR