Messages in this thread | | | Subject | Re: [PATCH -next] mm/filemap: fix that first page is not mark accessed in filemap_read() | From | Yu Kuai <> | Date | Mon, 6 Jun 2022 09:10:03 +0800 |
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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 > > . >
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