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?

So the idea was that if we're reading from a different _page_ that we read from previously, we should be marking it accessed. But there's an off by one error, it should have been

if (iocb->ki_pos >> PAGE_SHIFT != (ra->prev_pos - 1) >> PAGE_SHIFT)
	folio_mark_accessed(fbatch.folios[0])

It looks like this is what Yukai was arriving at too when he was saying ki_pos + copied - 1, this is just a cleaner way of writing it :)



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