Re: [PATCH 08/15] mm/filemap: add read support for RWF_UNCACHED

["Kirill A. Shutemov" <kirill@shutemov.name>]

On Mon, Nov 11, 2024 at 07:12:35AM -0700, Jens Axboe wrote:
> On 11/11/24 2:15 AM, Kirill A. Shutemov wrote:
> >> @@ -2706,8 +2712,16 @@ ssize_t filemap_read(struct kiocb *iocb, struct iov_iter *iter,
> >>  			}
> >>  		}
> >>  put_folios:
> >> -		for (i = 0; i < folio_batch_count(&fbatch); i++)
> >> -			folio_put(fbatch.folios[i]);
> >> +		for (i = 0; i < folio_batch_count(&fbatch); i++) {
> >> +			struct folio *folio = fbatch.folios[i];
> >> +
> >> +			if (folio_test_uncached(folio)) {
> >> +				folio_lock(folio);
> >> +				invalidate_complete_folio2(mapping, folio, 0);
> >> +				folio_unlock(folio);
> > 
> > I am not sure it is safe. What happens if it races with page fault?
> > 
> > The only current caller of invalidate_complete_folio2() unmaps the folio
> > explicitly before calling it. And folio lock prevents re-faulting.
> > 
> > I think we need to give up PG_uncached if we see folio_mapped(). And maybe
> > also mark the page accessed.
> 
> Ok thanks, let me take a look at that and create a test case that
> exercises that explicitly.

It might be worth generalizing it to clearing PG_uncached for any page cache
lookups that don't come from RWF_UNCACHED.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov