Re: [PATCH -next] mm/filemap: fix that first page is not mark accessed in filemap_read()
From: Kent Overstreet
Date: Fri Jun 10 2022 - 13:47:18 EST
On 6/10/22 10:36, Matthew Wilcox wrote:
On Fri, Jun 10, 2022 at 03:34:11PM +0100, Matthew Wilcox wrote:
On Mon, Jun 06, 2022 at 09:10:03AM +0800, Yu Kuai wrote:
On 2022/06/03 2:30, Matthew Wilcox wrote:
On Thu, Jun 02, 2022 at 04:21:29PM +0800, Yu Kuai wrote:
In filemap_read(), 'ra->prev_pos' is set to 'iocb->ki_pos + copied',
while it should be 'iocb->ki_ops'.
Can you walk me through your reasoning which leads you to believe that
it should be ki_pos instead of ki_pos + copied? As I understand it,
prev_pos is the end of the previous read, not the beginning of the
previous read.
Hi, Matthew
The main reason is the following judgement in flemap_read():
if (iocb->ki_pos >> PAGE_SHIFT != -> current page
ra->prev_pos >> PAGE_SHIFT) -> previous page
folio_mark_accessed(fbatch.folios[0]);
Which means if current page is the same as previous page, don't mark
page accessed. However, prev_pos is set to 'ki_pos + copied' during last
read, which will cause 'prev_pos >> PAGE_SHIFT' to be current page
instead of previous page.
I was thinking that if prev_pos is set to the begining of the previous
read, 'prev_pos >> PAGE_SHIFT' will be previous page as expected. Set to
the end of previous read is ok, however, I think the caculation of
previous page should be '(prev_pos - 1) >> PAGE_SHIFT' instead.
OK, I think Kent broke this in 723ef24b9b37 ("mm/filemap/c: break
generic_file_buffered_read up into multiple functions"). Before:
- prev_index = ra->prev_pos >> PAGE_SHIFT;
- prev_offset = ra->prev_pos & (PAGE_SIZE-1);
...
- if (prev_index != index || offset != prev_offset)
- mark_page_accessed(page);
After:
+ if (iocb->ki_pos >> PAGE_SHIFT != ra->prev_pos >> PAGE_SHIFT)
+ mark_page_accessed(page);
So surely this should have been:
+ if (iocb->ki_pos != ra->prev_pos)
+ mark_page_accessed(page);
Kent, do you recall why you changed it the way you did?
Oh, and if this is the right diagnosis, then this is the fix for the
current tree:
+++ b/mm/filemap.c
@@ -2673,8 +2673,7 @@ ssize_t filemap_read(struct kiocb *iocb, struct iov_iter *iter,
* When a sequential read accesses a page several times, only
* mark it as accessed the first time.
*/
- if (iocb->ki_pos >> PAGE_SHIFT !=
- ra->prev_pos >> PAGE_SHIFT)
+ if (iocb->ki_pos != ra->prev_pos)
folio_mark_accessed(fbatch.folios[0]);
for (i = 0; i < folio_batch_count(&fbatch); i++) {
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++) {