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[172.10.233.147]) by smtp.gmail.com with ESMTPSA id g14sm2323619qkk.38.2020.09.01.10.39.46 (version=TLS1 cipher=ECDHE-ECDSA-AES128-SHA bits=128/128); Tue, 01 Sep 2020 10:39:47 -0700 (PDT) Date: Tue, 1 Sep 2020 10:39:34 -0700 (PDT) From: Hugh Dickins X-X-Sender: hugh@eggly.anvils To: Yang Shi cc: Hugh Dickins , Andrew Morton , Alex Shi , Johannes Weiner , Michal Hocko , Mike Kravetz , Shakeel Butt , Matthew Wilcox , Qian Cai , Chris Wilson , Kuo-Hsin Yang , Linux Kernel Mailing List , Linux MM Subject: Re: [PATCH 3/5] shmem: shmem_writepage() split unlikely i915 THP In-Reply-To: Message-ID: References: User-Agent: Alpine 2.11 (LSU 23 2013-08-11) MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, 1 Sep 2020, Yang Shi wrote: > On Sun, Aug 30, 2020 at 2:04 PM Hugh Dickins 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 > > 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 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; > >