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, 13 Jun 2022 09:31:11 +0800 |
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在 2022/06/12 1:42, Matthew Wilcox 写道: > On Sat, Jun 11, 2022 at 04:23:42PM +0800, Yu Kuai wrote: >>> This is going to mark the folio as accessed multiple times if it's >>> a multi-page folio. How about this one? >>> >> Hi, Matthew >> >> Thanks for the patch, it looks good to me. > > Did you test it? This is clearly a little subtle ;-)
Yes, I confirmed that with this patch, small sequential read will mark page accessed. However, multi-page folio is not tested yet.
> >> BTW, I still think the fix should be commit 06c0444290ce ("mm/filemap.c: >> generic_file_buffered_read() now uses find_get_pages_contig"). > > Hmm, yes. That code also has problems, but they're more subtle and > probably don't amount to much. > > - iocb->ki_pos += copied; > - > - /* > - * 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) > - mark_page_accessed(page); > - > - ra->prev_pos = iocb->ki_pos; > > This will mark the page accessed when we _exit_ a page. So reading > 512-bytes at a time from offset 0, we'll mark page 0 as accessed on the > first read (because the prev_pos is initialised to -1). Then on the > eighth read, we'll mark page 0 as accessed again (because ki_pos will > now be 4096 and prev_pos is 3584). We'll then read chunks of page 1 > without marking it as accessed, until we're about to step into page 2.
You are right, I didn't think of that situation. > > Marking page 0 accessed twice is bad; it'll set the referenced bit the > first time, and then the second time, it'll activate it. So it'll be > thought to be part of the workingset when it's really just been part of > a streaming read. > > And the last page we read will never be marked accessed unless it > happens to finish at the end of a page. > > Before Kent started his refactoring, I think it worked: > > - pgoff_t prev_index; > - unsigned int prev_offset; > ... > - 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); > - prev_index = index; > - prev_offset = offset; > ... > - ra->prev_pos = prev_index; > - ra->prev_pos <<= PAGE_SHIFT; > - ra->prev_pos |= prev_offset; > > At least, I don't detect any bugs in this.
Sure, thanks for your explanation.
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