[PATCH 1/3] mm: Add a function to get a single tagged folio from a file

[David Howells <dhowells@redhat.com>]

Add a function to get a single tagged folio from a file rather than a batch
for use in afs and cifs where, in the common case, the batch is likely to
be rendered irrelevant by the {afs,cifs}_extend_writeback() function.

For filemap_get_folios_tag() to be of use, the batch has to be passed down,
and if it contains scattered, non-contiguous folios, these are likely to
end up being pinned by the batch for significant periods of time whilst I/O
is undertaken on earlier pages.

Further, for write_cache_pages() to be useful, it would need to wait for
PG_fscache which is used to indicate that I/O is in progress from a folio to
the cache - but it can't do this unconditionally as some filesystems, such
as btrfs, use PG_private_2 for other purposes.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Steve French <sfrench@samba.org>
cc: Linus Torvalds <torvalds@linux-foundation.org>
cc: "Vishal Moola (Oracle)" <vishal.moola@gmail.com>
cc: "Matthew Wilcox (Oracle)" <willy@infradead.org>
cc: Andrew Morton <akpm@linux-foundation.org>
cc: linux-afs@lists.infradead.org
cc: linux-cifs@vger.kernel.org
cc: linux-mm@kvack.org
Link: https://lore.kernel.org/r/2214157.1677250083@warthog.procyon.org.uk/
---
 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 0acb8e1fb7af..577535633006 100644
--- a/include/linux/pagemap.h
+++ b/include/linux/pagemap.h
@@ -741,6 +741,8 @@ 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);
+struct folio *filemap_get_folio_tag(struct address_space *mapping, pgoff_t *start,
+				    pgoff_t end, xa_mark_t tag);
 
 struct page *grab_cache_page_write_begin(struct address_space *mapping,
 			pgoff_t index);
diff --git a/mm/filemap.c b/mm/filemap.c
index 2723104cc06a..1b1e9c661018 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -2339,6 +2339,64 @@ unsigned filemap_get_folios_tag(struct address_space *mapping, pgoff_t *start,
 }
 EXPORT_SYMBOL(filemap_get_folios_tag);
 
+/**
+ * filemap_get_folio_tag - Get the first folio matching @tag
+ * @mapping:    The address_space to search
+ * @start:      The starting page index
+ * @end:        The final page index (inclusive)
+ * @tag:        The tag index
+ *
+ * Search for and return the first folios in the mapping starting at index
+ * @start and up to index @end (inclusive).  The folio is returned with an
+ * elevated reference count.
+ *
+ * If a folio is returned, it may start before @start; if it does, it will
+ * contain @start.  The folio may also extend beyond @end; if it does, it will
+ * contain @end.  If folios are added to or removed from the page cache while
+ * this is running, they may or may not be found by this call.
+ *
+ * Return: The folio that was found or NULL.  @start is also updated to index
+ * the next folio for the traversal or will be left pointing after @end.
+ */
+struct folio *filemap_get_folio_tag(struct address_space *mapping, pgoff_t *start,
+				    pgoff_t end, xa_mark_t tag)
+{
+	XA_STATE(xas, &mapping->i_pages, *start);
+	struct folio *folio;
+
+	rcu_read_lock();
+	while ((folio = find_get_entry(&xas, end, tag)) != NULL) {
+		/*
+		 * Shadow entries should never be tagged, but this iteration
+		 * is lockless so there is a window for page reclaim to evict
+		 * a page we saw tagged. Skip over it.
+		 */
+		if (xa_is_value(folio))
+			continue;
+
+		if (folio_test_hugetlb(folio))
+			*start = folio->index + 1;
+		else
+			*start = folio_next_index(folio);
+		goto out;
+	}
+
+	/*
+	 * We come here when there is no page 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 page 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;
+}
+EXPORT_SYMBOL(filemap_get_folio_tag);
+
 /*
  * 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: