From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 7C5EFEB64DA for ; Tue, 18 Jul 2023 09:34:06 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232318AbjGRJeF (ORCPT ); Tue, 18 Jul 2023 05:34:05 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:39364 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230479AbjGRJeD (ORCPT ); Tue, 18 Jul 2023 05:34:03 -0400 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by lindbergh.monkeyblade.net (Postfix) with ESMTP id ED51DE60 for ; Tue, 18 Jul 2023 02:34:01 -0700 (PDT) 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 26E332F4; Tue, 18 Jul 2023 02:34:45 -0700 (PDT) Received: from [10.1.34.52] (C02Z41KALVDN.cambridge.arm.com [10.1.34.52]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 4533C3F67D; Tue, 18 Jul 2023 02:34:00 -0700 (PDT) Message-ID: <225cbb00-9759-3547-8073-3b08b458c73e@arm.com> Date: Tue, 18 Jul 2023 10:33:58 +0100 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:102.0) Gecko/20100101 Thunderbird/102.13.0 Subject: Re: [PATCH v1 1/3] mm: Allow deferred splitting of arbitrary large anon folios To: David Hildenbrand , Andrew Morton , Matthew Wilcox , Yin Fengwei , Yu Zhao , Yang Shi , "Huang, Ying" , Zi Yan Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org References: <20230717143110.260162-1-ryan.roberts@arm.com> <20230717143110.260162-2-ryan.roberts@arm.com> <90b406af-9db4-b668-a7a0-e574e104c84c@redhat.com> <028c5f5b-b67c-9ccc-bc06-468f47362999@arm.com> <0250a2d7-c79b-0e0f-8161-bf475daf1c82@redhat.com> <7319a1aa-7c72-82e9-f26d-eeccb6fdf35b@arm.com> <7b0e691d-b224-20d0-a90a-bb659fbb3e1a@redhat.com> From: Ryan Roberts In-Reply-To: <7b0e691d-b224-20d0-a90a-bb659fbb3e1a@redhat.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 18/07/2023 10:08, David Hildenbrand wrote: > On 18.07.23 10:58, Ryan Roberts wrote: >> On 17/07/2023 17:48, David Hildenbrand wrote: >>> On 17.07.23 18:01, Ryan Roberts wrote: >>>> On 17/07/2023 16:42, David Hildenbrand wrote: >>>>> On 17.07.23 16:31, Ryan Roberts wrote: >>>>>> In preparation for the introduction of large folios for anonymous >>>>>> memory, we would like to be able to split them when they have unmapped >>>>>> subpages, in order to free those unused pages under memory pressure. So >>>>>> remove the artificial requirement that the large folio needed to be at >>>>>> least PMD-sized. >>>>>> >>>>>> Signed-off-by: Ryan Roberts >>>>>> Reviewed-by: Yu Zhao >>>>>> Reviewed-by: Yin Fengwei >>>>>> --- >>>>>>     mm/rmap.c | 2 +- >>>>>>     1 file changed, 1 insertion(+), 1 deletion(-) >>>>>> >>>>>> diff --git a/mm/rmap.c b/mm/rmap.c >>>>>> index 0c0d8857dfce..2baf57d65c23 100644 >>>>>> --- a/mm/rmap.c >>>>>> +++ b/mm/rmap.c >>>>>> @@ -1430,7 +1430,7 @@ void page_remove_rmap(struct page *page, struct >>>>>> vm_area_struct *vma, >>>>>>              * page of the folio is unmapped and at least one page >>>>>>              * is still mapped. >>>>>>              */ >>>>>> -        if (folio_test_pmd_mappable(folio) && folio_test_anon(folio)) >>>>>> +        if (folio_test_large(folio) && folio_test_anon(folio)) >>>>>>                 if (!compound || nr < nr_pmdmapped) >>>>>>                     deferred_split_folio(folio); >>>>> >>>>> !compound will always be true I guess, so nr_pmdmapped == 0 (which will always >>>>> be the case) will be ignored. >>>> >>>> I don't follow why !compound will always be true. This function is >>>> page_remove_rmap() (not folio_remove_rmap_range() which I add in a later >>>> patch). >>>> page_remove_rmap() can work on pmd-mapped pages where compound=true is >>>> passed in. >>> >>> I was talking about the folio_test_pmd_mappable() -> folio_test_large() change. >>> For folio_test_large() && !folio_test_pmd_mappable() I expect that we'll never >>> pass in "compound=true". >>> >> >> Sorry David, I've been staring at the code and your comment, and I still don't >> understand your point. I assumed you were trying to say that compound is always >> false and therefore "if (!compound || nr < nr_pmdmapped)" can be removed? But >> its not the case that compound is always false; it will be true when called to >> remove a pmd-mapped compound page. > > Let me try again: > > Assume, as I wrote, that we are given a folio that is "folio_test_large() && > !folio_test_pmd_mappable()". That is, a folio that is *not* pmd mappable. > > If it's not pmd-mappable, certainly, nr_pmdmapped == 0, and therefore, "nr < > nr_pmdmapped" will never ever trigger. > > The only way to have it added to the deferred split queue is, therefore "if > (!compound)". > > So *for these folios*, we will always pass "compound == false" to make that "if > (!compound)" succeed. > > > Does that make sense? Yes I agree with all of this. I thought you were pointing out an issue or proposing a change to the logic. Hence my confusion. > >> What change are you suggesting, exactly? > > Oh, I never suggested a change (I even gave you my RB). I was just thinking out > loud. >