Re: [PATCH 3/7] khugepaged: Drain LRU add pagevec to get rid of extra pins

[Kirill A. Shutemov]

On Fri, Mar 27, 2020 at 11:10:40AM -0700, Yang Shi wrote:
> On Fri, Mar 27, 2020 at 10:06 AM Kirill A. Shutemov
> <kirill@xxxxxxxxxxxxx> wrote:
> >
> > __collapse_huge_page_isolate() may fail due to extra pin in the LRU add
> > pagevec. It's petty common for swapin case: we swap in pages just to
> > fail due to the extra pin.
> >
> > Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> > ---
> >  mm/khugepaged.c | 8 ++++++++
> >  1 file changed, 8 insertions(+)
> >
> > diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> > index 14d7afc90786..39e0994abeb8 100644
> > --- a/mm/khugepaged.c
> > +++ b/mm/khugepaged.c
> > @@ -585,11 +585,19 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
> >                  * The page must only be referenced by the scanned process
> >                  * and page swap cache.
> >                  */
> > +               if (page_count(page) != 1 + PageSwapCache(page)) {
> > +                       /*
> > +                        * Drain pagevec and retry just in case we can get rid
> > +                        * of the extra pin, like in swapin case.
> > +                        */
> > +                       lru_add_drain();
> 
> This is definitely correct.
> 
> I'm wondering if we need one more lru_add_drain() before PageLRU check
> in khugepaged_scan_pmd() or not? The page might be in lru cache then
> get skipped. This would improve the success rate.

Could you elaborate on the scenario, I don't follow.


-- 
 Kirill A. Shutemov