mm: make pagecache tagged lookups return only head pages

Patch series "Overhaul multi-page lookups for THP", v4.

This THP prep patchset changes several page cache iteration APIs to only
return head pages.

 - It's only possible to tag head pages in the page cache, so only
   return head pages, not all their subpages.
 - Factor a lot of common code out of the various batch lookup routines
 - Add mapping_seek_hole_data()
 - Unify find_get_entries() and pagevec_lookup_entries()
 - Make find_get_entries only return head pages, like find_get_entry().

These are only loosely connected, but they seem to make sense together as
a series.

This patch (of 14):

Pagecache tags are used for dirty page writeback.  Since dirtiness is
tracked on a per-THP basis, we only want to return the head page rather
than each subpage of a tagged page.  All the filesystems which use huge
pages today are in-memory, so there are no tagged huge pages today.

Link: https://lkml.kernel.org/r/20201112212641.27837-2-willy@infradead.org
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: William Kucharski <william.kucharski@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Cc: Hugh Dickins <hughd@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Yang Shi <yang.shi@linux.alibaba.com>
Cc: Dave Chinner <dchinner@redhat.com>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Author: Matthew Wilcox (Oracle)

mm/filemap.c

@@ -2062,16 +2062,17 @@ unsigned find_get_pages_contig(struct address_space *mapping, pgoff_t index,
 EXPORT_SYMBOL(find_get_pages_contig);
 
 /**
- * find_get_pages_range_tag - find and return pages in given range matching @tag
+ * find_get_pages_range_tag - Find and return head pages matching @tag.
  * @mapping:	the address_space to search
  * @index:	the starting page index
  * @end:	The final page index (inclusive)
  * @tag:	the tag index
  * @nr_pages:	the maximum number of pages
  * @pages:	where the resulting pages are placed
  *
- * Like find_get_pages, except we only return pages which are tagged with
- * @tag.   We update @index to index the next page for the traversal.
+ * Like find_get_pages(), except we only return head pages which are tagged
+ * with @tag.  @index is updated to the index immediately after the last
+ * page we return, ready for the next iteration.
  *
  * Return: the number of pages which were found.
  */
@@ -2105,9 +2106,9 @@ unsigned find_get_pages_range_tag(struct address_space *mapping, pgoff_t *index,
 		if (unlikely(page != xas_reload(&xas)))
 			goto put_page;
 
-		pages[ret] = find_subpage(page, xas.xa_index);
+		pages[ret] = page;
 		if (++ret == nr_pages) {
-			*index = xas.xa_index + 1;
+			*index = page->index + thp_nr_pages(page);
 			goto out;
 		}
 		continue;