On Fri, Jun 10, 2022 at 01:47:02PM -0400, Kent Overstreet wrote:
> I think this is the fix we want - I think Yu basically had the right idea
> and had the off by one fix, this should be clearer though:
>
> Yu, can you confirm the fix?
>
> -- >8 --
> Subject: [PATCH] filemap: Fix off by one error when marking folios accessed
>
> In filemap_read() we mark pages accessed as we read them - but we don't
> want to do so redundantly, if the previous read already did so.
>
> But there was an off by one error: we want to check if the current page
> was the same as the last page we read from, but the last page we read
> from was (ra->prev_pos - 1) >> PAGE_SHIFT.
>
> Reported-by: Yu Kuai <yukuai3@xxxxxxxxxx>
> Signed-off-by: Kent Overstreet <kent.overstreet@xxxxxxxxx>
>
> diff --git a/mm/filemap.c b/mm/filemap.c
> index 9daeaab360..8d5c8043cb 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -2704,7 +2704,7 @@ ssize_t filemap_read(struct kiocb *iocb, struct
> iov_iter *iter,
> * mark it as accessed the first time.
> */
> if (iocb->ki_pos >> PAGE_SHIFT !=
> - ra->prev_pos >> PAGE_SHIFT)
> + (ra->prev_pos - 1) >> PAGE_SHIFT)
> folio_mark_accessed(fbatch.folios[0]);
>
> for (i = 0; i < folio_batch_count(&fbatch); i++) {
>
This is going to mark the folio as accessed multiple times if it's
a multi-page folio. How about this one?
diff --git a/mm/filemap.c b/mm/filemap.c
index 5f227b5420d7..a30587f2e598 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -2599,6 +2599,13 @@ static int filemap_get_pages(struct kiocb *iocb, struct iov_iter *iter,
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.
@@ -2670,11 +2677,11 @@ 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++) {
