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[172.10.233.147]) by smtp.gmail.com with ESMTPSA id w10-20020a05690204ea00b00b7767ca747csm3435131ybs.25.2023.04.17.21.41.42 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 17 Apr 2023 21:41:43 -0700 (PDT) Date: Mon, 17 Apr 2023 21:41:41 -0700 (PDT) From: Hugh Dickins X-X-Sender: hugh@ripple.attlocal.net To: Luis Chamberlain cc: hughd@google.com, akpm@linux-foundation.org, willy@infradead.org, brauner@kernel.org, linux-mm@kvack.org, p.raghav@samsung.com, da.gomez@samsung.com, a.manzanares@samsung.com, dave@stgolabs.net, yosryahmed@google.com, keescook@chromium.org, patches@lists.linux.dev, linux-kernel@vger.kernel.org, David Hildenbrand Subject: Re: [PATCH v2 4/6] shmem: skip page split if we're not reclaiming In-Reply-To: <20230309230545.2930737-5-mcgrof@kernel.org> Message-ID: References: <20230309230545.2930737-1-mcgrof@kernel.org> <20230309230545.2930737-5-mcgrof@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, 9 Mar 2023, Luis Chamberlain wrote: > In theory when info->flags & VM_LOCKED we should not be getting > shem_writepage() called so we should be verifying this with a > WARN_ON_ONCE(). Since we should not be swapping then best to ensure > we also don't do the folio split earlier too. So just move the check > early to avoid folio splits in case its a dubious call. > > We also have a similar early bail when !total_swap_pages so just move > that earlier to avoid the possible folio split in the same situation. > > Acked-by: David Hildenbrand > Reviewed-by: Christian Brauner > Signed-off-by: Luis Chamberlain > --- > mm/shmem.c | 10 ++++++---- > 1 file changed, 6 insertions(+), 4 deletions(-) > > diff --git a/mm/shmem.c b/mm/shmem.c > index 68e9970baf1e..dfd995da77b4 100644 > --- a/mm/shmem.c > +++ b/mm/shmem.c > @@ -1350,6 +1350,12 @@ static int shmem_writepage(struct page *page, struct writeback_control *wbc) > if (WARN_ON_ONCE(!wbc->for_reclaim)) > goto redirty; > > + if (WARN_ON_ONCE(info->flags & VM_LOCKED)) > + goto redirty; Well, okay, I don't mind that. But shall we take bets on how soon syzbot (hope it's not watching) will try flipping SHM_LOCK on while swapping out pages from a SHM segment, and hit that warning? Perhaps I'm wrong, but I don't think any serialization prevents that. Hugh > + > + if (!total_swap_pages) > + goto redirty; > + > /* > * If /sys/kernel/mm/transparent_hugepage/shmem_enabled is "always" or > * "force", drivers/gpu/drm/i915/gem/i915_gem_shmem.c gets huge pages, > @@ -1365,10 +1371,6 @@ static int shmem_writepage(struct page *page, struct writeback_control *wbc) > } > > index = folio->index; > - if (info->flags & VM_LOCKED) > - goto redirty; > - if (!total_swap_pages) > - goto redirty; > > /* > * This is somewhat ridiculous, but without plumbing a SWAP_MAP_FALLOC > -- > 2.39.1