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 E4E7351C5D for ; Mon, 18 Mar 2024 15:36:55 +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=1710776218; cv=none; b=TpK/1fnAwAE+QJIs8emR3wqbheltQ6xW/TAfc/mb5TjMumtZtx/++S7nKHEITpIRBx0vtMgXYFuVupEBPbO/3aSTgkSCyd3VK3aPPbhgVIoMeEepEsWMsAc63TXqsXEi9lFOrZ/H/e13WApnbriuq8TH9yerRVYgKsmUA+Y2zRA= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1710776218; c=relaxed/simple; bh=/y6gO1eFPOe9gs3dFKb6yOEbShwj17hfm0c183BvHLo=; h=Message-ID:Date:MIME-Version:Subject:From:To:Cc:References: In-Reply-To:Content-Type; b=ejAg1eR8U4HgJLH6ylFMTSRRhRv2M8j2peOXf7ghAm9xJk5ZyhGRbeW4+pCDY7eU+b65N4tk26DqlL4xecHS+Xm25DnZMTlAzh4nkqn3WgYMhQ+xTAAHnciW92+NEMn8QB08/P4FyKq1BybTKlSA/j5ZPVIF0NDn5iQOeQTl1Us= 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 6A34CDA7; Mon, 18 Mar 2024 08:37:30 -0700 (PDT) Received: from [10.57.68.51] (unknown [10.57.68.51]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id E2E2A3F67D; Mon, 18 Mar 2024 08:36:52 -0700 (PDT) Message-ID: <8696d022-32fe-4888-80c6-87f6a0ac70a9@arm.com> Date: Mon, 18 Mar 2024 15:36:52 +0000 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v4 5/6] mm: vmscan: Avoid split during shrink_folio_list() Content-Language: en-GB From: Ryan Roberts To: David Hildenbrand , "Yin, Fengwei" , "Huang, Ying" Cc: "linux-mm@kvack.org" , "linux-kernel@vger.kernel.org" , Andrew Morton , Matthew Wilcox , Gao Xiang , Yu Zhao , Yang Shi , Michal Hocko , Kefeng Wang , Barry Song <21cnbao@gmail.com>, Chris Li References: <20240311150058.1122862-1-ryan.roberts@arm.com> <20240311150058.1122862-6-ryan.roberts@arm.com> <1db242d3-5ff1-4ef5-b20a-578a317fa859@arm.com> <5cc147fa-492e-46c3-963d-2f6e6680dce9@arm.com> <878r2gwahg.fsf@yhuang6-desk2.ccr.corp.intel.com> In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit On 18/03/2024 15:35, Ryan Roberts wrote: > On 18/03/2024 10:05, David Hildenbrand wrote: >> On 18.03.24 11:00, Yin, Fengwei wrote: >>> >>> >>> On 3/18/2024 10:16 AM, Huang, Ying wrote: >>>> Ryan Roberts writes: >>>> >>>>> Hi Yin Fengwei, >>>>> >>>>> On 15/03/2024 11:12, David Hildenbrand wrote: >>>>>> On 15.03.24 11:49, Ryan Roberts wrote: >>>>>>> On 15/03/2024 10:43, David Hildenbrand wrote: >>>>>>>> On 11.03.24 16:00, Ryan Roberts wrote: >>>>>>>>> Now that swap supports storing all mTHP sizes, avoid splitting large >>>>>>>>> folios before swap-out. This benefits performance of the swap-out path >>>>>>>>> by eliding split_folio_to_list(), which is expensive, and also sets us >>>>>>>>> up for swapping in large folios in a future series. >>>>>>>>> >>>>>>>>> If the folio is partially mapped, we continue to split it since we want >>>>>>>>> to avoid the extra IO overhead and storage of writing out pages >>>>>>>>> uneccessarily. >>>>>>>>> >>>>>>>>> Signed-off-by: Ryan Roberts >>>>>>>>> --- >>>>>>>>>      mm/vmscan.c | 9 +++++---- >>>>>>>>>      1 file changed, 5 insertions(+), 4 deletions(-) >>>>>>>>> >>>>>>>>> diff --git a/mm/vmscan.c b/mm/vmscan.c >>>>>>>>> index cf7d4cf47f1a..0ebec99e04c6 100644 >>>>>>>>> --- a/mm/vmscan.c >>>>>>>>> +++ b/mm/vmscan.c >>>>>>>>> @@ -1222,11 +1222,12 @@ static unsigned int shrink_folio_list(struct >>>>>>>>> list_head >>>>>>>>> *folio_list, >>>>>>>>>                          if (!can_split_folio(folio, NULL)) >>>>>>>>>                              goto activate_locked; >>>>>>>>>                          /* >>>>>>>>> -                     * Split folios without a PMD map right >>>>>>>>> -                     * away. Chances are some or all of the >>>>>>>>> -                     * tail pages can be freed without IO. >>>>>>>>> +                     * Split partially mapped folios map >>>>>>>>> +                     * right away. Chances are some or all >>>>>>>>> +                     * of the tail pages can be freed >>>>>>>>> +                     * without IO. >>>>>>>>>                           */ >>>>>>>>> -                    if (!folio_entire_mapcount(folio) && >>>>>>>>> +                    if (!list_empty(&folio->_deferred_list) && >>>>>>>>>                              split_folio_to_list(folio, >>>>>>>>>                                      folio_list)) >>>>>>>>>                              goto activate_locked; >>>>>>>> >>>>>>>> Not sure if we might have to annotate that with data_race(). >>>>>>> >>>>>>> I asked that exact question to Matthew in another context bt didn't get a >>>>>>> response. There are examples of checking if the deferred list is empty >>>>>>> with and >>>>>>> without data_race() in the code base. But list_empty() is implemented like >>>>>>> this: >>>>>>> >>>>>>> static inline int list_empty(const struct list_head *head) >>>>>>> { >>>>>>>       return READ_ONCE(head->next) == head; >>>>>>> } >>>>>>> >>>>>>> So I assumed the READ_ONCE() makes everything safe without a lock? Perhaps >>>>>>> not >>>>>>> sufficient for KCSAN? >>> I don't think READ_ONCE() can replace the lock. > > But it doesn't ensure we get a consistent value and that the compiler orders the Sorry - fat fingers... I meant it *does* ensure we get a consistent value (i.e. untorn) > load correctly. There are lots of patterns in the kernel that use READ_ONCE() > without a lock and they don't use data_race() - e.g. ptep_get_lockless(). > > It sounds like none of us really understand what data_race() is for, so I guess > I'll just do a KCSAN build and invoke the code path to see if it complains. > > >>> >>>>>> >>>>>> Yeah, there is only one use of data_race with that list. >>>>>> >>>>>> It was added in f3ebdf042df4 ("THP: avoid lock when check whether THP is in >>>>>> deferred list"). >>>>>> >>>>>> Looks like that was added right in v1 of that change [1], so my best guess is >>>>>> that it is not actually required. >>>>>> >>>>>> If not required, likely we should just cleanup the single user. >>>>>> >>>>>> [1] >>>>>> https://lore.kernel.org/linux-mm/20230417075643.3287513-2-fengwei.yin@intel.com/ >>>>> >>>>> Do you have any recollection of why you added the data_race() markup? >>>> >>>> Per my understanding, this is used to mark that the code accesses >>>> folio->_deferred_list without lock intentionally, while >>>> folio->_deferred_list may be changed in parallel.  IIUC, this is what >>>> data_race() is used for.  Or, my understanding is wrong? >>> Yes. This is my understanding also. >> >> Why don't we have a data_race() in deferred_split_folio() then, before taking >> the lock? >> >> It's used a bit inconsistently here. >> >