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@xxxxxxxxxx>
cc: Steve French <sfrench@xxxxxxxxx>
cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
cc: "Vishal Moola (Oracle)" <vishal.moola@xxxxxxxxx>
cc: "Matthew Wilcox (Oracle)" <willy@xxxxxxxxxxxxx>
cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
cc: linux-afs@xxxxxxxxxxxxxxxxxxx
cc: linux-cifs@xxxxxxxxxxxxxxx
cc: linux-mm@xxxxxxxxx
Link: https://lore.kernel.org/r/2214157.1677250083@xxxxxxxxxxxxxxxxxxxxxx/
---
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:
