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[195.135.220.15]) by mx.google.com with ESMTPS id e31si684753ede.102.2019.04.17.05.07.38 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Wed, 17 Apr 2019 05:07:38 -0700 (PDT) Received-SPF: pass (google.com: domain of vbabka@suse.cz designates 195.135.220.15 as permitted sender) client-ip=195.135.220.15; Authentication-Results: mx.google.com; spf=pass (google.com: domain of vbabka@suse.cz designates 195.135.220.15 as permitted sender) smtp.mailfrom=vbabka@suse.cz X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.220.254]) by mx1.suse.de (Postfix) with ESMTP id 7AF31AFFC; Wed, 17 Apr 2019 12:07:37 +0000 (UTC) Subject: Re: [PATCH] mm: fix false-positive OVERCOMMIT_GUESS failures To: Roman Gushchin , Johannes Weiner Cc: Andrew Morton , "linux-mm@kvack.org" , "linux-kernel@vger.kernel.org" , Kernel Team References: <20190412191418.26333-1-hannes@cmpxchg.org> <20190412200629.GA24377@tower.DHCP.thefacebook.com> From: Vlastimil Babka Openpgp: preference=signencrypt Autocrypt: addr=vbabka@suse.cz; 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Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.6.1 MIME-Version: 1.0 In-Reply-To: <20190412200629.GA24377@tower.DHCP.thefacebook.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit 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 4/12/19 10:06 PM, Roman Gushchin wrote: > On Fri, Apr 12, 2019 at 03:14:18PM -0400, Johannes Weiner wrote: >> With the default overcommit==guess we occasionally run into mmap >> rejections despite plenty of memory that would get dropped under >> pressure but just isn't accounted reclaimable. One example of this is >> dying cgroups pinned by some page cache. A previous case was auxiliary >> path name memory associated with dentries; we have since annotated >> those allocations to avoid overcommit failures (see d79f7aa496fc ("mm: >> treat indirectly reclaimable memory as free in overcommit logic")). >> >> But trying to classify all allocated memory reliably as reclaimable >> and unreclaimable is a bit of a fool's errand. There could be a myriad >> of dependencies that constantly change with kernel versions. Just wondering, did you find at least one another reclaimable case like those path names? >> It becomes even more questionable of an effort when considering how >> this estimate of available memory is used: it's not compared to the >> system-wide allocated virtual memory in any way. It's not even >> compared to the allocating process's address space. It's compared to >> the single allocation request at hand! >> >> So we have an elaborate left-hand side of the equation that tries to >> assess the exact breathing room the system has available down to a >> page - and then compare it to an isolated allocation request with no >> additional context. We could fail an allocation of N bytes, but for >> two allocations of N/2 bytes we'd do this elaborate dance twice in a >> row and then still let N bytes of virtual memory through. This doesn't >> make a whole lot of sense. >> >> Let's take a step back and look at the actual goal of the >> heuristic. From the documentation: >> >> Heuristic overcommit handling. Obvious overcommits of address >> space are refused. Used for a typical system. It ensures a >> seriously wild allocation fails while allowing overcommit to >> reduce swap usage. root is allowed to allocate slightly more >> memory in this mode. This is the default. >> >> If all we want to do is catch clearly bogus allocation requests >> irrespective of the general virtual memory situation, the physical >> memory counter-part doesn't need to be that complicated, either. >> >> When in GUESS mode, catch wild allocations by comparing their request >> size to total amount of ram and swap in the system. >> >> Signed-off-by: Johannes Weiner > > My 2c here: any kinds of percpu counters and percpu data is accounted > as unreclaimable and can alter the calculation significantly. > > This is a special problem on hosts, which were idle for some time. > Without any memory pressure, kernel caches do occupy most of the memory, > so than a following attempt to start a workload fails. So then we remove the kmalloc-reclaimable caches again as not worth the trouble anymore (they might be useful for anti-fragmentation purposes, but that's much harder to quantify), or what? > With a big pleasure: > Acked-by: Roman Gushchin > > Thanks! >