Re: Removing ->dirty_folio

[John Hubbard <jhubbard@nvidia.com>]

On 8/24/26 12:43 PM, Matthew Wilcox wrote:
> 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

oh, I just thought of a minor hole that we need to fill: how to mark the
page dirty in the first place, in the absence of mark_[page|folio]_dirty()?

Under this new scheme, we will need to pin first, then mark dirty, to
set up. The filesystem can't do everything, because even if it were to
call page_mkdirty(), a writeback could clear that before the page gets
pinned.

Then teardown is simply to unpin, as discussed already.

>> 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) >=

As long as the math works: need to not underflow. I guess mapcount is
always less than refcount, so OK.

So it *seems* correct to me, fwiw. :)

>                 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.

thanks,
-- 
John Hubbard