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

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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++) {




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