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

[Jinchao Wang]

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()