Re: [PATCH -next] mm/filemap: fix that first page is not mark accessed in filemap_read()

[Matthew Wilcox <willy@infradead.org>]

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@huawei.com>
> Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>
> 
> 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++) {