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

[Matthew Wilcox <willy@infradead.org>]

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.