Messages in this thread | | | Date | Fri, 10 Jun 2022 15:34:11 +0100 | From | Matthew Wilcox <> | Subject | Re: [PATCH -next] mm/filemap: fix that first page is not mark accessed in filemap_read() |
| |
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?
| |