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Wed, 1 Jul 2020 14:46:40 +0000 (UTC) IronPort-SDR: eyP3cSPZdCgY6fRfG6JX0sFEXxvMV8AZyX7y9B7rBnR6DOq6IPi6cOMwSiTqxttD+lZEL66vQ+ ChlXkvqKRVEQ== X-IronPort-AV: E=McAfee;i="6000,8403,9668"; a="126213983" X-IronPort-AV: E=Sophos;i="5.75,300,1589266800"; d="scan'208";a="126213983" X-Amp-Result: SKIPPED(no attachment in message) X-Amp-File-Uploaded: False Received: from orsmga005.jf.intel.com ([10.7.209.41]) by fmsmga106.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 01 Jul 2020 07:46:39 -0700 IronPort-SDR: 0D34c0Aa1lNpj2xS2p9Zk/J0b43lpi67c0hU1c4oJ+rrSuZGTG2Wi0NiEu9VDmPUEv8/3JQ0df 7XBfitWJDuag== X-IronPort-AV: E=Sophos;i="5.75,300,1589266800"; d="scan'208";a="455128380" Received: from rapyeatx-mobl3.amr.corp.intel.com (HELO [10.255.2.31]) ([10.255.2.31]) by orsmga005-auth.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 01 Jul 2020 07:46:36 -0700 Subject: Re: [RFC][PATCH 2/8] mm/migrate: Defer allocating new page until needed To: Greg Thelen , Dave Hansen , linux-kernel@vger.kernel.org Cc: linux-mm@kvack.org, kbusch@kernel.org, yang.shi@linux.alibaba.com, rientjes@google.com, ying.huang@intel.com, dan.j.williams@intel.com References: <20200629234503.749E5340@viggo.jf.intel.com> <20200629234507.CA0FDE19@viggo.jf.intel.com> From: Dave Hansen Autocrypt: addr=dave.hansen@intel.com; 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Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.8.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US X-Rspamd-Queue-Id: 4E21D1814B0D2 X-Spamd-Result: default: False [0.00 / 100.00] X-Rspamd-Server: rspam05 Content-Transfer-Encoding: quoted-printable 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 7/1/20 1:47 AM, Greg Thelen wrote: > Dave Hansen wrote: >> From: Keith Busch >> Defer allocating the page until we are actually ready to make use of >> it, after locking the original page. This simplifies error handling, >> but should not have any functional change in behavior. This is just >> refactoring page migration so the main part can more easily be reused >> by other code. >=20 > Is there any concern that the src page is now held PG_locked over the > dst page allocation, which might wander into > reclaim/cond_resched/oom_kill? I don't have a deadlock in mind. I'm > just wondering about the additional latency imposed on unrelated thread= s > who want access src page. It's not great. *But*, the alternative is to toss the page contents out and let users encounter a fault and an allocation. They would be subject to all the latency associated with an allocation, just at a slightly later time. If it's a problem it seems like it would be pretty easy to fix, at least for non-cgroup reclaim. We know which node we're reclaiming from and we know if it has a demotion path, so we could proactively allocate a single migration target page before doing the source lock_page(). That creates some other problems, but I think it would be straightforward. >> #Signed-off-by: Keith Busch >=20 > Is commented Signed-off-by intentional? Same applies to later patches. Yes, Keith is no longer at Intel, so that @intel.com mail would bounce. I left the @intel.com SoB so it would be clear that the code originated from Keith while at Intel, but commented it out to avoid it being picked up by anyone's tooling.