linux-kernel - Re: [PATCH 3/5] shmem: shmem_writepage() split unlikely i915 THP

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Date:   Tue, 1 Sep 2020 11:50:06 -0700
From:   Yang Shi <shy828301@...il.com>
To:     Hugh Dickins <hughd@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Alex Shi <alex.shi@...ux.alibaba.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...e.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Shakeel Butt <shakeelb@...gle.com>,
        Matthew Wilcox <willy@...radead.org>, Qian Cai <cai@....pw>,
        Chris Wilson <chris@...is-wilson.co.uk>,
        Kuo-Hsin Yang <vovoy@...omium.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>
Subject: Re: [PATCH 3/5] shmem: shmem_writepage() split unlikely i915 THP

On Tue, Sep 1, 2020 at 10:39 AM Hugh Dickins <hughd@...gle.com> wrote:
>
> On Tue, 1 Sep 2020, Yang Shi wrote:
> > On Sun, Aug 30, 2020 at 2:04 PM Hugh Dickins <hughd@...gle.com> wrote:
> > >
> > > drivers/gpu/drm/i915/gem/i915_gem_shmem.c contains a shmem_writeback()
> > > which calls shmem_writepage() from a shrinker: that usually works well
> > > enough; but if /sys/kernel/mm/transparent_hugepage/shmem_enabled has
> > > been set to "force" (documented as "Force the huge option on for all -
> > > very useful for testing"), shmem_writepage() is surprised to be called
> > > with a huge page, and crashes on the VM_BUG_ON_PAGE(PageCompound) (I
> > > did not find out where the crash happens when CONFIG_DEBUG_VM is off).
> > >
> > > LRU page reclaim always splits the shmem huge page first: I'd prefer not
> > > to demand that of i915, so check and split compound in shmem_writepage().
> > >
> > > Fixes: 2d6692e642e7 ("drm/i915: Start writeback from the shrinker")
> > > Signed-off-by: Hugh Dickins <hughd@...gle.com>
> > > Cc: stable@...r.kernel.org # v5.3+
> > > ---
> > > I've marked this for stable just for the info, but the number of users
> > > affected is very probably 1, so please feel free to delete that marking.
> > >
> > >  mm/shmem.c |    9 +++++++++
> > >  1 file changed, 9 insertions(+)
> > >
> > > --- 5.9-rc2/mm/shmem.c  2020-08-16 17:32:50.693507198 -0700
> > > +++ linux/mm/shmem.c    2020-08-28 17:35:08.326024349 -0700
> > > @@ -1362,7 +1362,15 @@ static int shmem_writepage(struct page *
> > >         swp_entry_t swap;
> > >         pgoff_t index;
> > >
> > > -       VM_BUG_ON_PAGE(PageCompound(page), page);
> > > +       /*
> > > +        * If /sys/kernel/mm/transparent_hugepage/shmem_enabled is "force",
> > > +        * then drivers/gpu/drm/i915/gem/i915_gem_shmem.c gets huge pages,
> > > +        * and its shmem_writeback() needs them to be split when swapping.
> > > +        */
> > > +       if (PageTransCompound(page))
> > > +               if (split_huge_page(page) < 0)
> > > +                       goto redirty;
> >
> > The change looks good to me. Acked-by: Yang Shi <shy828301@...il.com>
>
> Thanks.
>
> >
> > Just a nit: it may be better to move the spilte after the !PageLocked
> > assertion? Split needs page locked too.
>
> I hadn't considered that, but I think it's best left as is:
> split_huge_page_to_list() has its own
>         VM_BUG_ON_PAGE(!PageLocked(head), head);
> to enforce its needs: think of the old BUG_ON(!PageLocked(page))
> below as enforcing shmem's needs, checking that split_huge_page()
> did not unlock it :)

Yes, it is definitely fine to keep it as is. I just thought we could
bailout earlier if the page is not locked.

>
> >
> > > +
> > >         BUG_ON(!PageLocked(page));
> > >         mapping = page->mapping;
> > >         index = page->index;
> > >