Re: [PATCH v18 31/32] mm: Add explicit page decrement in exception path for isolate_lru_pages

[Alexander Duyck]

On Tue, Sep 8, 2020 at 6:01 PM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
>
> On Mon, Aug 24, 2020 at 08:55:04PM +0800, Alex Shi wrote:
> > +++ b/mm/vmscan.c
> > @@ -1688,10 +1688,13 @@ static unsigned long isolate_lru_pages(unsigned long nr_to_scan,
> >
> >                       if (!TestClearPageLRU(page)) {
> >                               /*
> > -                              * This page may in other isolation path,
> > -                              * but we still hold lru_lock.
> > +                              * This page is being isolated in another
> > +                              * thread, but we still hold lru_lock. The
> > +                              * other thread must be holding a reference
> > +                              * to the page so this should never hit a
> > +                              * reference count of 0.
> >                                */
> > -                             put_page(page);
> > +                             WARN_ON(put_page_testzero(page));
> >                               goto busy;
>
> I read Hugh's review and that led me to take a look at this.  We don't
> do it like this.  Use the same pattern as elsewhere in mm:
>
>         page_ref_sub(page, nr);
>         VM_BUG_ON_PAGE(page_count(page) <= 0, page);
>
>

Actually for this case page_ref_dec(page) would make more sense
wouldn't it? Otherwise I agree that would be a better change if that
is the way it has been handled before. I just wasn't familiar with
those other spots.

Thanks.

- Alex