From 5ccc944dce3df5fd2fd683a7df4fd49d1068eba2 Mon Sep 17 00:00:00 2001 From: "Matthew Wilcox (Oracle)" Date: Fri, 10 Jun 2022 14:44:41 -0400 Subject: [PATCH] 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 Reviewed-by: Kent Overstreet Signed-off-by: Matthew Wilcox (Oracle) --- mm/filemap.c | 13 ++++++++++--- 1 file changed, 10 insertions(+), 3 deletions(-) diff --git a/mm/filemap.c b/mm/filemap.c index ac3775c1ce4c..577068868449 100644 --- 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++) { -- 2.34.1