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 :)
