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.
For consequence,
folio_mark_accessed() will not be called for 'fbatch.folios[0]' since
'iocb->ki_pos' is always equal to 'ra->prev_pos'.
I don't follow this, but maybe I'm just being slow.
I mssing a condition here:
Under small io sequential read, folio_mark_accessed() will not be called
for 'fbatch.folios[0]' since 'iocb->ki_pos' is always equal to
'ra->prev_pos'.
Thanks,
Kuai
.
