Messages in this thread | | | Date | Tue, 1 Sep 2020 10:39:34 -0700 (PDT) | From | Hugh Dickins <> | Subject | Re: [PATCH 3/5] shmem: shmem_writepage() split unlikely i915 THP |
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On Tue, 1 Sep 2020, Yang Shi wrote: > On Sun, Aug 30, 2020 at 2:04 PM Hugh Dickins <hughd@google.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@google.com> > > Cc: stable@vger.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@gmail.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 :)
> > > + > > BUG_ON(!PageLocked(page)); > > mapping = page->mapping; > > index = page->index; > >
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