Messages in this thread | | | Date | Fri, 10 Jun 2022 13:23:21 -0400 | Subject | Re: [PATCH -next] mm/filemap: fix that first page is not mark accessed in filemap_read() | From | Kent Overstreet <> |
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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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