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

[Alexander Duyck]

On Wed, Sep 9, 2020 at 11:24 AM Hugh Dickins <hughd@xxxxxxxxxx> wrote:
>
> On Wed, 9 Sep 2020, 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.
>
> After overnight reflection, my own preference would be simply to
> drop this patch.  I think we are making altogether too much of a
> fuss here over what was simply correct as plain put_page()
> (and further from correct if we change it to leak the page in an
> unforeseen circumstance).
>
> And if Alex's comment was not quite grammatically correct, never mind,
> it said as much as was worth saying.  I got more worried by his
> placement of the "busy:" label, but that does appear to work correctly.
>
> There's probably a thousand places where put_page() is used, where
> it would be troublesome if it were the final put_page(): this one
> bothered you because you'd been looking at isolate_migratepages_block(),
> and its necessary avoidance of lru_lock recursion on put_page();
> but let's just just leave this put_page() as is.

I'd be fine with that, but I would still like to see the comment
updated. At a minimum we should make it clear that we believe that
put_page is safe here as it should never reach zero and if it does
then we are looking at a bug. Then if this starts triggering soft
lockups  we at least have documentation somewhere that someone can
reference on what we expected and why we triggered a lockup.

- Alex