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s=arc-20220608; d=hostedemail.com; t=1723442433; a=rsa-sha256; cv=none; b=xtHnOXAhfCfoj2fpLw2K0Odam3gMR3wWXHCghRvkS424n/qYiMmMFlrsYeWcQYQpAF0yTM nI9YAK3CrjBOz9SBY/QdG/PhyGAjpMheuPX4ryfwDM0ac6ocPBX1yinWj4x1DgqihboPXF xS32ZB6yYPxUHdPFNNwBJjStk96rVxY= ARC-Authentication-Results: i=1; imf06.hostedemail.com; dkim=none; spf=pass (imf06.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 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 D2334FEC; Sun, 11 Aug 2024 23:02:07 -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 81B9B3F73B; Sun, 11 Aug 2024 23:01:32 -0700 (PDT) Message-ID: <15dbe4ac-a036-4029-ba08-e12a236f448a@arm.com> Date: Mon, 12 Aug 2024 11:31:29 +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> Content-Language: en-US From: Dev Jain In-Reply-To: <87frrauwwv.fsf@yhuang6-desk2.ccr.corp.intel.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Stat-Signature: d1u34wtgb3z9psybusw7khsimydgp1tr X-Rspamd-Queue-Id: 3289418002B X-Rspam-User: X-Rspamd-Server: rspam08 X-HE-Tag: 1723442502-755756 X-HE-Meta: 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 vNJf/B5u MPApXTRc91nwowXm1g/pzckciWe2mfFTFsUf7CJiy5/FNfQ4aCEKgSa/dGf5OA/ObyinMb/Xd9gnhA76LDr151i4zIJSzfB9vP6R5oFh6NUK+S2O312g1zmVku93zDM7RGmQ9OBB/SNzJIptgm/ITodc46+h45pKXbV73yRUqZBeHomoDXghuk3xlVDSB0ePlQvtCGmy0j6P4eKY= 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 11:04, 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. I'll get back to you on this. > > 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. As per my understanding, this is not the current behaviour for sync migration. After successful unmapping, we fail in migrate_folio_move() with -EAGAIN, we do not call undo src+dst (rendering the loop around migrate_folio_move() futile), we do not push the failed folio onto the ret_folios list, therefore, in _sync(), _batch() is never tried again. > > -- > Best Regards, > Huang, Ying