Re: [RFC PATCH v2 3/5] mm: Add explicit page decrement in exception path for isolate_lru_pages

[Alexander Duyck]

On Wed, Aug 19, 2020 at 12:52 AM Alex Shi <alex.shi@xxxxxxxxxxxxxxxxx> wrote:
>
>
>
> 在 2020/8/19 下午12:27, Alexander Duyck 写道:
> > From: Alexander Duyck <alexander.h.duyck@xxxxxxxxxxxxxxx>
> >
> > In isolate_lru_pages we have an exception path where if we call
> > get_page_unless_zero and that succeeds, but TestClearPageLRU fails we call
> > put_page. Normally this would be problematic but due to the way that the
> > calls are ordered and the fact that we are holding the LRU lock we know
> > that the caller must be holding another reference for the page. Since we
> > can assume that we can replace the put_page with a call to
> > put_page_testzero contained within a WARN_ON. By doing this we should see
> > if we ever leak a page as a result of the reference count somehow hitting
> > zero when it shouldn't, and can avoid the overhead and confusion of using
> > the full put_page call.
> >
> > Signed-off-by: Alexander Duyck <alexander.h.duyck@xxxxxxxxxxxxxxx>
> > ---
> >  mm/vmscan.c |    9 ++++++---
> >  1 file changed, 6 insertions(+), 3 deletions(-)
> >
> > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > index 5bc0c2322043..3ebe3f9b653b 100644
> > --- a/mm/vmscan.c
> > +++ 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));
>
> seems WARN_ON is always enabled.
>
> Reviewed-by: Alex Shi <alex.shi@xxxxxxxxxxxxxxxxx>

Yeah, it is always enabled however it should never be triggered. I had
considered just putting a page_ref_dec here since in theory this path
should never be triggered but I thought as a debug catch I add the
WARN_ON and put_page_testzero. If we ever do encounter this being
triggered then it will leak a page of memory which isn't the end of
the world but I thought would warrant a WARN_ON.