Messages in this thread Patch in this message | | | Date | Fri, 10 Jun 2022 13:47:02 -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? > > Oh, and if this is the right diagnosis, then this is the fix for the > current tree: > > +++ b/mm/filemap.c > @@ -2673,8 +2673,7 @@ ssize_t filemap_read(struct kiocb *iocb, struct iov_iter *iter, > * When a sequential read accesses a page several times, only > * mark it as accessed the first time. > */ > - if (iocb->ki_pos >> PAGE_SHIFT != > - ra->prev_pos >> PAGE_SHIFT) > + if (iocb->ki_pos != ra->prev_pos) > folio_mark_accessed(fbatch.folios[0]); > > for (i = 0; i < folio_batch_count(&fbatch); i++) { > >
I think this is the fix we want - I think Yu basically had the right idea and had the off by one fix, this should be clearer though:
Yu, can you confirm the fix?
-- >8 -- Subject: [PATCH] filemap: Fix off by one error when marking folios accessed
In filemap_read() we mark pages accessed as we read them - but we don't want to do so redundantly, if the previous read already did so.
But there was an off by one error: we want to check if the current page was the same as the last page we read from, but the last page we read from was (ra->prev_pos - 1) >> PAGE_SHIFT.
Reported-by: Yu Kuai <yukuai3@huawei.com> Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>
diff --git a/mm/filemap.c b/mm/filemap.c index 9daeaab360..8d5c8043cb 100644 --- a/mm/filemap.c +++ b/mm/filemap.c @@ -2704,7 +2704,7 @@ ssize_t filemap_read(struct kiocb *iocb, struct iov_iter *iter, * mark it as accessed the first time. */ if (iocb->ki_pos >> PAGE_SHIFT != - ra->prev_pos >> PAGE_SHIFT) + (ra->prev_pos - 1) >> PAGE_SHIFT) folio_mark_accessed(fbatch.folios[0]);
for (i = 0; i < folio_batch_count(&fbatch); i++) {
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