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imf10.hostedemail.com; dkim=none; spf=pass (imf10.hostedemail.com: domain of dev.jain@arm.com designates 217.140.110.172 as permitted sender) smtp.mailfrom=dev.jain@arm.com; dmarc=pass (policy=none) header.from=arm.com ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1723444334; a=rsa-sha256; cv=none; b=ahw2BuDnv30ynOzTscvu5Fs/SvPyo9C8R2UjMVMIdCsj9+xEOYCqR/48izVLkcaCZtSFg5 o5t79SQ4LRwMySMVjYXQaIJhDMsgiqX8xNYjaHb1VyvQ39mLSomnO0lI7KMw7xYLRfBgVh NVYN049sjE8HyZWY+wynihWZxWQeyHw= Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id D5D4EFEC; Sun, 11 Aug 2024 23:33:33 -0700 (PDT) Received: from [10.162.43.141] (e116581.arm.com [10.162.43.141]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id A59D33F73B; Sun, 11 Aug 2024 23:32:58 -0700 (PDT) Message-ID: Date: Mon, 12 Aug 2024 12:02:55 +0530 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 1/2] mm: Retry migration earlier upon refcount mismatch To: "Huang, Ying" Cc: akpm@linux-foundation.org, shuah@kernel.org, david@redhat.com, willy@infradead.org, ryan.roberts@arm.com, anshuman.khandual@arm.com, catalin.marinas@arm.com, cl@gentwo.org, vbabka@suse.cz, mhocko@suse.com, apopple@nvidia.com, osalvador@suse.de, baolin.wang@linux.alibaba.com, dave.hansen@linux.intel.com, will@kernel.org, baohua@kernel.org, ioworker0@gmail.com, gshan@redhat.com, mark.rutland@arm.com, kirill.shutemov@linux.intel.com, hughd@google.com, aneesh.kumar@kernel.org, yang@os.amperecomputing.com, peterx@redhat.com, broonie@kernel.org, mgorman@techsingularity.net, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-kselftest@vger.kernel.org References: <20240809103129.365029-1-dev.jain@arm.com> <20240809103129.365029-2-dev.jain@arm.com> <87frrauwwv.fsf@yhuang6-desk2.ccr.corp.intel.com> <877ccmuus2.fsf@yhuang6-desk2.ccr.corp.intel.com> Content-Language: en-US From: Dev Jain In-Reply-To: <877ccmuus2.fsf@yhuang6-desk2.ccr.corp.intel.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Stat-Signature: eb6z8ysmcc3mjxg9iwzwbnfj3jpfpcjk X-Rspam-User: X-Rspamd-Queue-Id: CB2FFC001F X-Rspamd-Server: rspam02 X-HE-Tag: 1723444388-487330 X-HE-Meta: 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 tSwRF6Py d7TqnHKFf7hsT4SNh3F8Pz826AUyyMp/vfxS+F3WUV0WMOu/visWLCFWRo6Bz7GobTF9z1nV5cchOgDi5PlkzHcYY+WnJL3ZWqhLp3mntxFMWKRXKk8WzEDV/ANbyoPrBnQS0VhO23uFZ7EBFupuQPcKjOurve6CGK2GvsVq/FHyqjpr2TRakVOqIEowDYW/wdIuPYCGKfpMtNRbf19h6hGwx9YzZxq3NdIa/kD3bcoZm4doURhRi7q4hLZggzIjlc4wpfdVLN9xvMWI= 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: List-Subscribe: List-Unsubscribe: On 8/12/24 11:50, Huang, Ying wrote: > Dev Jain writes: > >> On 8/12/24 11:04, Huang, Ying wrote: >>> Hi, Dev, >>> >>> Dev Jain writes: >>> >>>> As already being done in __migrate_folio(), wherein we backoff if the >>>> folio refcount is wrong, make this check during the unmapping phase, upon >>>> the failure of which, the original state of the PTEs will be restored and >>>> the folio lock will be dropped via migrate_folio_undo_src(), any racing >>>> thread will make progress and migration will be retried. >>>> >>>> Signed-off-by: Dev Jain >>>> --- >>>> mm/migrate.c | 9 +++++++++ >>>> 1 file changed, 9 insertions(+) >>>> >>>> diff --git a/mm/migrate.c b/mm/migrate.c >>>> index e7296c0fb5d5..477acf996951 100644 >>>> --- a/mm/migrate.c >>>> +++ b/mm/migrate.c >>>> @@ -1250,6 +1250,15 @@ static int migrate_folio_unmap(new_folio_t get_new_folio, >>>> } >>>> if (!folio_mapped(src)) { >>>> + /* >>>> + * Someone may have changed the refcount and maybe sleeping >>>> + * on the folio lock. In case of refcount mismatch, bail out, >>>> + * let the system make progress and retry. >>>> + */ >>>> + struct address_space *mapping = folio_mapping(src); >>>> + >>>> + if (folio_ref_count(src) != folio_expected_refs(mapping, src)) >>>> + goto out; >>>> __migrate_folio_record(dst, old_page_state, anon_vma); >>>> return MIGRATEPAGE_UNMAP; >>>> } >>> Do you have some test results for this? For example, after applying the >>> patch, the migration success rate increased XX%, etc. >> Noting that the migration selftest is operating on a single page, >> before the patch, the test fails on shared-anon mappings on an >> average of 10 iterations of move_pages(), and after applying the >> patch it fails on average of 100 iterations, which makes sense >> because the unmapping() will get retried 3 + 7 = 10 times. > Thanks! What is the test results for > > https://lore.kernel.org/all/20240801081657.1386743-1-dev.jain@arm.com/ > > ? That solves the problem completely, makes the test pass. Although that still may not be a good solution since it solves the problem for this particular case (reader thread faulting and raising refcount). As David notes, a concurrent read()/write() should also create this refcount race problem. > >>> My understanding for this issue is that the migration success rate can >>> increase if we undo all changes before retrying. This is the current >>> behavior for sync migration, but not for async migration. If so, we can >>> use migrate_pages_sync() for async migration too to increase success >>> rate? Of course, we need to change the function name and comments. > -- > Best Regards, > Huang, Ying >