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Tue, 24 Aug 2021 14:04:36 +0000 (UTC) Received: from dovecot-director2.suse.de ([192.168.254.65]) by imap1.suse-dmz.suse.de with ESMTPSA id 0HQrOXT8JGHfOwAAGKfGzw (envelope-from ); Tue, 24 Aug 2021 14:04:36 +0000 Message-ID: <2f034e6f-a753-550a-f374-e4e23899d3d5@suse.cz> Date: Tue, 24 Aug 2021 16:04:36 +0200 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.0.1 Subject: Re: [PATCH] mm, vmscan: guarantee drop_slab_node() termination Content-Language: en-US To: Matthew Wilcox , Andrew Morton Cc: linux-mm@kvack.org, Muchun Song , Chris Down , Michal Hocko , Chunxin Zang References: <20210818152239.25502-1-vbabka@suse.cz> From: Vlastimil Babka In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Authentication-Results: imf08.hostedemail.com; dkim=pass header.d=suse.cz header.s=susede2_rsa header.b=M76Dvocu; dkim=pass header.d=suse.cz header.s=susede2_ed25519 header.b=kKdEpnix; dmarc=none; spf=pass (imf08.hostedemail.com: domain of vbabka@suse.cz designates 195.135.220.29 as permitted sender) smtp.mailfrom=vbabka@suse.cz X-Rspamd-Server: rspam06 X-Rspamd-Queue-Id: B58BA30000B1 X-Stat-Signature: dfo4yrf8nkx6zqtf5qs54szyctaxsxff X-HE-Tag: 1629813878-162677 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On 8/24/21 12:02, Matthew Wilcox wrote: > On Wed, Aug 18, 2021 at 05:22:39PM +0200, Vlastimil Babka wrote: >> diff --git a/mm/vmscan.c b/mm/vmscan.c >> index 403a175a720f..ef3554314b47 100644 >> --- a/mm/vmscan.c >> +++ b/mm/vmscan.c >> @@ -936,6 +936,7 @@ static unsigned long shrink_slab(gfp_t gfp_mask, int nid, >> void drop_slab_node(int nid) >> { >> unsigned long freed; >> + int shift = 0; >> >> do { >> struct mem_cgroup *memcg = NULL; >> @@ -948,7 +949,7 @@ void drop_slab_node(int nid) >> do { >> freed += shrink_slab(GFP_KERNEL, nid, memcg, 0); >> } while ((memcg = mem_cgroup_iter(NULL, memcg, NULL)) != NULL); >> - } while (freed > 10); >> + } while ((freed >> shift++) > 0); > > This can, if you're really unlucky, produce UB. If you free 2^63 items > when shift is 63, then 2^63 >> 63 is 1 and shift becomes 64, producing > UB on the next iteration. We could do: > > } while (shift < BITS_PER_LONG) && (freed >> shift++) > 0); > > but honestly, that feels silly. How about: > > } while ((freed >> shift++) > 1); > > almost exactly as arbitrary, but guarantees no UB. Hey, zero is not arbitrary :P But thanks, here's a fix up. >From 88189bf16406c5910400193422b3f18f859f18d8 Mon Sep 17 00:00:00 2001 From: Vlastimil Babka Date: Tue, 24 Aug 2021 14:08:53 +0200 Subject: [PATCH] mm, vmscan: guarantee drop_slab_node() termination-fix Matthew reports [1] that if we free enough objects, we can eventually right-shift by BITS_PER_LONG, which is undefined behavior. Raise the threshold from 0 to 1 which means we will shift only up to BITS_PER_LONG-1. [1] https://lore.kernel.org/linux-mm/YSTDnqKgQLvziyQI@casper.infradead.org/ Reported-by: Matthew Wilcox Signed-off-by: Vlastimil Babka --- mm/vmscan.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/mm/vmscan.c b/mm/vmscan.c index 4ffaa7970904..f08aef08c351 100644 --- a/mm/vmscan.c +++ b/mm/vmscan.c @@ -952,7 +952,7 @@ void drop_slab_node(int nid) do { freed += shrink_slab(GFP_KERNEL, nid, memcg, 0); } while ((memcg = mem_cgroup_iter(NULL, memcg, NULL)) != NULL); - } while ((freed >> shift++) > 0); + } while ((freed >> shift++) > 1); } void drop_slab(void) -- 2.32.0