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SubjectRe: [PATCH v7 5/6] mm,thp: add read-only THP support for (non-shmem) FS
On Mon, Jun 24, 2019 at 03:04:21PM +0000, Song Liu wrote:
>
>
> > On Jun 24, 2019, at 7:54 AM, Kirill A. Shutemov <kirill@shutemov.name> wrote:
> >
> > On Mon, Jun 24, 2019 at 02:42:13PM +0000, Song Liu wrote:
> >>
> >>
> >>> On Jun 24, 2019, at 7:27 AM, Kirill A. Shutemov <kirill@shutemov.name> wrote:
> >>>
> >>> On Mon, Jun 24, 2019 at 02:01:05PM +0000, Song Liu wrote:
> >>>>>> @@ -1392,6 +1403,23 @@ static void collapse_file(struct mm_struct *mm,
> >>>>>> result = SCAN_FAIL;
> >>>>>> goto xa_unlocked;
> >>>>>> }
> >>>>>> + } else if (!page || xa_is_value(page)) {
> >>>>>> + xas_unlock_irq(&xas);
> >>>>>> + page_cache_sync_readahead(mapping, &file->f_ra, file,
> >>>>>> + index, PAGE_SIZE);
> >>>>>> + lru_add_drain();
> >>>>>
> >>>>> Why?
> >>>>
> >>>> isolate_lru_page() is likely to fail if we don't drain the pagevecs.
> >>>
> >>> Please add a comment.
> >>
> >> Will do.
> >>
> >>>
> >>>>>> + page = find_lock_page(mapping, index);
> >>>>>> + if (unlikely(page == NULL)) {
> >>>>>> + result = SCAN_FAIL;
> >>>>>> + goto xa_unlocked;
> >>>>>> + }
> >>>>>> + } else if (!PageUptodate(page)) {
> >>>>>
> >>>>> Maybe we should try wait_on_page_locked() here before give up?
> >>>>
> >>>> Are you referring to the "if (!PageUptodate(page))" case?
> >>>
> >>> Yes.
> >>
> >> I think this case happens when another thread is reading the page in.
> >> I could not think of a way to trigger this condition for testing.
> >>
> >> On the other hand, with current logic, we will retry the page on the
> >> next scan, so I guess this is OK.
> >
> > What I meant that calling wait_on_page_locked() on !PageUptodate() page
> > will likely make it up-to-date and we don't need to SCAN_FAIL the attempt.
> >
>
> Yeah, I got the point. My only concern is that I don't know how to
> reliably trigger this case for testing. I can try to trigger it. But I
> don't know whether it will happen easily.

Atrifically slowing down IO should do the trick.

--
Kirill A. Shutemov

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