mm/page-writeback: document folio_mark_dirty() locking more explicitly

We have had bugs where set_page_dirty() was used on a page from GUP
without appropriate locking, leading to UAF, in:

 - KVM, see
   https://lore.kernel.org/r/20260810-x86-kvm-setpagedirty-v1-1-85f180892d4f@google.com
 - i915, see commit 0d4bbe3d407f ("drm/i915/userptr: Try to acquire the
   page lock around set_page_dirty()").
 - VMCI, see commit 5a16c535409f ("VMCI: Use set_page_dirty_lock() when
   unregistering guest memory")
 - kpc2000 staging driver, see commit b6d13bd9f2c1 ("staging: kpc2000:
   kpc_dma: Convert set_page_dirty() --> set_page_dirty_lock()")

I think set_page_dirty() and folio_mark_dirty() need more explicit
documentation on how they should be used with pages from GUP; so add a
comment on top of set_page_dirty() and make the comment above
folio_mark_dirty() more explicit.

Link: https://lore.kernel.org/20260810-set-page-dirty-warnings-v2-1-1bd40fadfacd@google.com
Signed-off-by: Jann Horn <jannh@google.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

0 files changed, 0 insertions, 0 deletions