Re: [PATCH v3 1/7] filemap: add helper to look up dirty folios in a range

Darrick J. Wong
2025-07-15T05:20:15Z

On Mon, Jul 14, 2025 at 04:41:16PM -0400, Brian Foster wrote:
> Add a new filemap_get_folios_dirty() helper to look up existing dirty
> folios in a range and add them to a folio_batch. This is to support
> optimization of certain iomap operations that only care about dirty
> folios in a target range. For example, zero range only zeroes the subset
> of dirty pages over unwritten mappings, seek hole/data may use similar
> logic in the future, etc.
>
> Note that the helper is intended for use under internal fs locks.
> Therefore it trylocks folios in order to filter out clean folios.
> This loosely follows the logic from filemap_range_has_writeback().
>
> Signed-off-by: Brian Foster <bfoster@redhat.com>
> Reviewed-by: Christoph Hellwig <hch@lst.de>

This seems correct to me, though like hch said, I'd like to hear from
willy.
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>

--D

> ---
> include/linux/pagemap.h | 2 ++
> mm/filemap.c | 58 +++++++++++++++++++++++++++++++++++++++++
> 2 files changed, 60 insertions(+)
>
> diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
> index e63fbfbd5b0f..fb83ddf26621 100644
> --- a/include/linux/pagemap.h
> +++ b/include/linux/pagemap.h
> @@ -941,6 +941,8 @@ unsigned filemap_get_folios_contig(struct address_space *mapping,
> pgoff_t *start, pgoff_t end, struct folio_batch *fbatch);
> unsigned filemap_get_folios_tag(struct address_space *mapping, pgoff_t *start,
> pgoff_t end, xa_mark_t tag, struct folio_batch *fbatch);
> +unsigned filemap_get_folios_dirty(struct address_space *mapping,
> + pgoff_t *start, pgoff_t end, struct folio_batch *fbatch);
>
> /*
> * Returns locked page at given index in given cache, creating it if needed.
> diff --git a/mm/filemap.c b/mm/filemap.c
> index bada249b9fb7..2171b7f689b0 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -2334,6 +2334,64 @@ unsigned filemap_get_folios_tag(struct address_space *mapping, pgoff_t *start,
> }
> EXPORT_SYMBOL(filemap_get_folios_tag);
>
> +/**
> + * filemap_get_folios_dirty - Get a batch of dirty folios
> + * @mapping: The address_space to search
> + * @start: The starting folio index
> + * @end: The final folio index (inclusive)
> + * @fbatch: The batch to fill
> + *
> + * filemap_get_folios_dirty() works exactly like filemap_get_folios(), except
> + * the returned folios are presumed to be dirty or undergoing writeback. Dirty
> + * state is presumed because we don't block on folio lock nor want to miss
> + * folios. Callers that need to can recheck state upon locking the folio.
> + *
> + * This may not return all dirty folios if the batch gets filled up.
> + *
> + * Return: The number of folios found.
> + * Also update @start to be positioned for traversal of the next folio.
> + */
> +unsigned filemap_get_folios_dirty(struct address_space *mapping, pgoff_t *start,
> + pgoff_t end, struct folio_batch *fbatch)
> +{
> + XA_STATE(xas, &mapping->i_pages, *start);
> + struct folio *folio;
> +
> + rcu_read_lock();
> + while ((folio = find_get_entry(&xas, end, XA_PRESENT)) != NULL) {
> + if (xa_is_value(folio))
> + continue;
> + if (folio_trylock(folio)) {
> + bool clean = !folio_test_dirty(folio) &&
> + !folio_test_writeback(folio);
> + folio_unlock(folio);
> + if (clean) {
> + folio_put(folio);
> + continue;
> + }
> + }
> + if (!folio_batch_add(fbatch, folio)) {
> + unsigned long nr = folio_nr_pages(folio);
> + *start = folio->index + nr;
> + goto out;
> + }
> + }
> + /*
> + * We come here when there is no folio beyond @end. We take care to not
> + * overflow the index @start as it confuses some of the callers. This
> + * breaks the iteration when there is a folio at index -1 but that is
> + * already broke anyway.
> + */
> + if (end == (pgoff_t)-1)
> + *start = (pgoff_t)-1;
> + else
> + *start = end + 1;
> +out:
> + rcu_read_unlock();
> +
> + return folio_batch_count(fbatch);
> +}
> +
> /*
> * CD/DVDs are error prone. When a medium error occurs, the driver may fail
> * a _large_ part of the i/o request. Imagine the worst scenario:
> --
> 2.50.0
>
>