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

[Matthew Wilcox]

On Wed, Sep 09, 2020 at 08:43:38AM -0700, Alexander Duyck wrote:
> 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.

Yes, page_ref_dec() should be fine.  It's hard to remember which of
VM_BUG_ON, WARN_ON, etc, compile down to nothing with various CONFIG
options, and which ones actually evalauate their arguments.  Safer not
to put things with side-effects inside macros.