From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by smtp.subspace.kernel.org (Postfix) with ESMTP id 4E7CF17A582; Mon, 12 Aug 2024 12:08:47 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=217.140.110.172 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1723464530; cv=none; b=YL5ptIRO+qtPVgPYkIY6SagePkvAD67qOf0UFY4yGXzaGZ84iXaBBXhE+cPemjA9LKM31pUoBy6Er8HfsrV63Gltmoc+8aYYTsIhc5YXQNzQo8FZd/sJmaxHEl52PFwNvGKA7r3r5V0joxl9fDPfmZZe3F7t0kxElHKyvPRAjvc= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1723464530; c=relaxed/simple; bh=rvKqs+ZnBTanZvIsHe7cLaM3f9ejiJKlAUKbE98Z0ik=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=NXccsdDLmbeuz7oeJxhmSfH0j378OB5PbqO5ZqLoeVxCave/ByNtgW8vETkae9BBoi9L7wmE1cfXUTY7KzVqPlfwSGFZg8Wd3+IJRnWz6KTyy2wDU8dwIK9JI5YtbbXIsPNIFQHaNU6jdGKZLclh88lNm+V7u+ZxsRUSmrX8Xo0= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com; spf=pass smtp.mailfrom=arm.com; arc=none smtp.client-ip=217.140.110.172 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=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 4D770FEC; Mon, 12 Aug 2024 05:09:13 -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 3E9013F73B; Mon, 12 Aug 2024 05:08:38 -0700 (PDT) Message-ID: <9d84e4e8-ac54-4eb1-a113-3f32aea864c9@arm.com> Date: Mon, 12 Aug 2024 17:38:35 +0530 Precedence: bulk X-Mailing-List: linux-kselftest@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: 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> <15dbe4ac-a036-4029-ba08-e12a236f448a@arm.com> <87bk1yuuzu.fsf@yhuang6-desk2.ccr.corp.intel.com> <95b72817-5444-4ced-998a-1cb90f42bf49@arm.com> <8734naurhm.fsf@yhuang6-desk2.ccr.corp.intel.com> Content-Language: en-US From: Dev Jain In-Reply-To: <8734naurhm.fsf@yhuang6-desk2.ccr.corp.intel.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 8/12/24 13:01, Huang, Ying wrote: > Dev Jain writes: > >> On 8/12/24 11:45, 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. >>>> 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. >>> In migrate_pages_sync(), migrate_pages_batch(,MIGRATE_ASYNC) will be >>> called first, if failed, the folio will be restored to the original >>> state (unlocked). Then migrate_pages_batch(,_SYNC*) is called again. >>> So, we unlock once. If it's necessary, we can unlock more times via >>> another level of loop. >> Yes, that's my point. We need to undo src+dst and retry. > For sync migration, we undo src+dst and retry now, but only once. You > have shown that more retrying increases success rate. > >> We will have >> to decide where we want this retrying to be; do we want to change the >> return value, end up in the while loop wrapped around _sync(), and retry >> there by adding another level of loop, or do we want to make use of the >> existing retry loops, one of which is wrapped around _unmap(); the latter >> is my approach. The utility I see for the former approach is that, in case >> of a large number of page migrations (which should usually be the case), >> we are giving more time for the folio to get retried. The latter does not >> give much time and discards the folio if it did not succeed under 7 times. > Because it's a race, I guess that most folios will be migrated > successfully in the first pass. > > My concerns of your method are that it deal with just one case > specially. While retrying after undoing all appears more general. Makes sense. Also, please ignore my "change the return value" thing, I got confused between unmap_folios, ret_folios, etc. Now I think I understood what the lists are doing :) > > If it's really important to retry after undoing all, we can either > convert two retying loops of migrate_pages_batch() into one loop, or > remove retry loop in migrate_pages_batch() and retry in its caller > instead. And if I implemented this correctly, the following makes the test pass always: https://www.codedump.xyz/diff/Zrn7EdxzNXmXyNXe > > -- > Best Regards, > Huang, Ying