Re: Removing ->dirty_folio

[Matthew Wilcox <willy@infradead.org>]

On Mon, Aug 24, 2026 at 12:25:42PM -0700, John Hubbard wrote:
> > My proposal is this:
> > 
> >  - Fileystems take note of folio_maybe_dma_pinned() during writeback.
> >    If it's true, do the writeback, but retain/recreate whatever data
> >    structures you need in order to write the folio again; behave as if
> >    ->page_mkdirty() had been called again for each page in the folio is
> >    marked as dirty.
> 
> Yes, that would work nicely.
> 
> >  - The MM behaves similarly; we do not clear the writeback flag for
> >    folio_maybe_dma_pinned().
> > 
> > This will have the effect of writing pinned folios back every time the
> > inode is scheduled for writeback.  But since we have no idea whether
> > the folio is actually dirty (because the GUP user won't tell us),
> > this is the correct behaviour.
> > 
> > I'm probably missing some stuff here.  Let me know.
> 
> OK, so working through the end of the pinning, I think it still is
> correct: device finishes writing to pinned memory, device driver
> unpins the memory but the page has been left marked dirty the whole
> time and still is, so the next writeback still does the writeback,
> but this time sees no pins and so it marks the page clean.

Excellent!  By the way, what would you think to this?

@@ -2717,7 +2719,8 @@ static inline bool folio_maybe_dma_pinned(struct folio *folio)
         * Here, for that overflow case, use the sign bit to count a little
         * bit higher via unsigned math, and thus still get an accurate result.
         */
-       return ((unsigned int)folio_ref_count(folio)) >=
+       mapcount = folio_mapcount(folio);
+       return (folio_ref_count(folio) - mapcount) >=
                GUP_PIN_COUNTING_BIAS;
 }


It should improve the accuracy of folio_maybe_dma_pinned() for folios
which are mapped many, many times (eg a page of libc).  I'm a little
concerned about races turning that number negative since we don't
necessarily have the folio locked at that point.