linux-kernel - Re: [PATCH] mm/readahead: read min folio constraints under invalidate lock

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Message-ID: <aUDXrYgwZAMYkXVu@casper.infradead.org>
Date: Tue, 16 Dec 2025 03:53:17 +0000
From: Matthew Wilcox <willy@...radead.org>
To: Jinchao Wang <wangjinchao600@...il.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
	Christian Brauner <brauner@...nel.org>,
	Hannes Reinecke <hare@...e.de>,
	Luis Chamberlain <mcgrof@...nel.org>, linux-fsdevel@...r.kernel.org,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	stable@...r.kernel.org,
	syzbot+4d3cc33ef7a77041efa6@...kaller.appspotmail.com,
	syzbot+fdba5cca73fee92c69d6@...kaller.appspotmail.com
Subject: Re: [PATCH] mm/readahead: read min folio constraints under
 invalidate lock

On Tue, Dec 16, 2025 at 11:12:21AM +0800, Jinchao Wang wrote:
> On Tue, Dec 16, 2025 at 02:42:06AM +0000, Matthew Wilcox wrote:
> > On Tue, Dec 16, 2025 at 09:37:51AM +0800, Jinchao Wang wrote:
> > > On Mon, Dec 15, 2025 at 02:22:23PM +0000, Matthew Wilcox wrote:
> > > > On Mon, Dec 15, 2025 at 10:19:00PM +0800, Jinchao Wang wrote:
> > > > > page_cache_ra_order() and page_cache_ra_unbounded() read mapping minimum folio
> > > > > constraints before taking the invalidate lock, allowing concurrent changes to
> > > > > violate page cache invariants.
> > > > > 
> > > > > Move the lookups under filemap_invalidate_lock_shared() to ensure readahead
> > > > > allocations respect the mapping constraints.
> > > > 
> > > > Why are the mapping folio size constraints being changed?  They're
> > > > supposed to be set at inode instantiation and then never changed.
> > > 
> > > They can change after instantiation for block devices. In the syzbot repro:
> > >   blkdev_ioctl() -> blkdev_bszset() -> set_blocksize() ->
> > >   mapping_set_folio_min_order()
> > 
> > Oh, this is just syzbot doing stupid things.  We should probably make
> > blkdev_bszset() fail if somebody else has an fd open.
> 
> Thanks, that makes sense.
> Tightening blkdev_bszset() would avoid the race entirely.
> This change is meant as a defensive fix to prevent BUGs.

Yes, but the point is that there's a lot of code which relies on
the AS_FOLIO bits not changing in the middle.  Syzbot found one of them,
but there are others.