filemap: Correct the conditions for marking a folio as accessed

We had an off-by-one error which meant that we never marked the first page
in a read as accessed.  This was visible as a slowdown when re-reading
a file as pages were being evicted from cache too soon.  In reviewing
this code, we noticed a second bug where a multi-page folio would be
marked as accessed multiple times when doing reads that were less than
the size of the folio.

Abstract the comparison of whether two file positions are in the same
folio into a new function, fixing both of these bugs.

Reported-by: Yu Kuai <yukuai3@huawei.com>
Reviewed-by: Kent Overstreet <kent.overstreet@gmail.com>
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>

diff --git a/mm/filemap.c b/mm/filemap.c
index ac3775c1ce4cd853857d8838d3a86f087f3b9aa4..5770688684492e69e514c9f0556ae5e1c6345954 100644 (file)
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -2629,6 +2629,13 @@ err:
        return err;
 }
 
+static inline bool pos_same_folio(loff_t pos1, loff_t pos2, struct folio *folio)
+{
+       unsigned int shift = folio_shift(folio);
+
+       return (pos1 >> shift == pos2 >> shift);
+}
+
 /**
  * filemap_read - Read data from the page cache.
  * @iocb: The iocb to read.
@@ -2700,11 +2707,11 @@ ssize_t filemap_read(struct kiocb *iocb, struct iov_iter *iter,
                writably_mapped = mapping_writably_mapped(mapping);
 
                /*
-                * When a sequential read accesses a page several times, only
+                * When a read accesses the same folio several times, only
                 * mark it as accessed the first time.
                 */
-               if (iocb->ki_pos >> PAGE_SHIFT !=
-                   ra->prev_pos >> PAGE_SHIFT)
+               if (!pos_same_folio(iocb->ki_pos, ra->prev_pos - 1,
+                                                       fbatch.folios[0]))
                        folio_mark_accessed(fbatch.folios[0]);
 
                for (i = 0; i < folio_batch_count(&fbatch); i++) {