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 DFA1DC433EF for ; Sat, 16 Apr 2022 02:34:12 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229980AbiDPCgk (ORCPT ); Fri, 15 Apr 2022 22:36:40 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:45850 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229948AbiDPCgh (ORCPT ); Fri, 15 Apr 2022 22:36:37 -0400 Received: from szxga02-in.huawei.com (szxga02-in.huawei.com [45.249.212.188]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 378C064BC6 for ; Fri, 15 Apr 2022 19:34:05 -0700 (PDT) Received: from canpemm500002.china.huawei.com (unknown [172.30.72.54]) by szxga02-in.huawei.com (SkyGuard) with ESMTP id 4KgHJS5QV8zFprd; Sat, 16 Apr 2022 10:31:36 +0800 (CST) Received: from [10.174.177.76] (10.174.177.76) by canpemm500002.china.huawei.com (7.192.104.244) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) id 15.1.2375.24; Sat, 16 Apr 2022 10:34:03 +0800 Subject: Re: [PATCH v2 2/9] mm/vmscan: remove unneeded can_split_huge_page check To: "ying.huang@intel.com" , David Hildenbrand , Oscar Salvador CC: , , , , , , Pavel Tatashin , John Hubbard , Linus Torvalds , Vlastimil Babka , Yu Zhao References: <20220409093500.10329-1-linmiaohe@huawei.com> <20220409093500.10329-3-linmiaohe@huawei.com> <7455b680-3d89-5d3e-ba0e-6e4358b114a2@huawei.com> <4b47e6317aca3deeabf610a7f4839563ff2b25a1.camel@intel.com> From: Miaohe Lin Message-ID: <70b64476-6efc-c8ad-9cf3-b101c3b92db1@huawei.com> Date: Sat, 16 Apr 2022 10:34:02 +0800 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:78.0) Gecko/20100101 Thunderbird/78.6.0 MIME-Version: 1.0 In-Reply-To: <4b47e6317aca3deeabf610a7f4839563ff2b25a1.camel@intel.com> Content-Type: text/plain; charset="utf-8" Content-Language: en-US Content-Transfer-Encoding: 7bit X-Originating-IP: [10.174.177.76] X-ClientProxiedBy: dggems706-chm.china.huawei.com (10.3.19.183) To canpemm500002.china.huawei.com (7.192.104.244) X-CFilter-Loop: Reflected Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2022/4/15 11:07, ying.huang@intel.com wrote: > On Wed, 2022-04-13 at 09:26 +0800, ying.huang@intel.com wrote: >> On Tue, 2022-04-12 at 16:59 +0200, David Hildenbrand wrote: >>> On 12.04.22 15:42, Miaohe Lin wrote: >>>> On 2022/4/12 16:59, Oscar Salvador wrote: >>>>> On Sat, Apr 09, 2022 at 05:34:53PM +0800, Miaohe Lin wrote: >>>>>> We don't need to check can_split_folio() because folio_maybe_dma_pinned() >>>>>> is checked before. It will avoid the long term pinned pages to be swapped >>>>>> out. And we can live with short term pinned pages. Without can_split_folio >>>>>> checking we can simplify the code. Also activate_locked can be changed to >>>>>> keep_locked as it's just short term pinning. >>>>> >>>>> What do you mean by "we can live with short term pinned pages"? >>>>> Does it mean that it was not pinned when we check >>>>> folio_maybe_dma_pinned() but now it is? >>>>> >>>>> To me it looks like the pinning is fluctuating and we rely on >>>>> split_folio_to_list() to see whether we succeed or not, and if not >>>>> we give it another spin in the next round? >>>> >>>> Yes. Short term pinned pages is relative to long term pinned pages and these pages won't be >>>> pinned for a noticeable time. So it's expected to split the folio successfully in the next >>>> round as the pinning is really fluctuating. Or am I miss something? >>>> >>> >>> Just so we're on the same page. folio_maybe_dma_pinned() only capture >>> FOLL_PIN, but not FOLL_GET. You can have long-term FOLL_GET right now >>> via vmsplice(). >> >> Per my original understanding, folio_maybe_dma_pinned() can be used to >> detect long-term pinned pages. And it seems reasonable to skip the >> long-term pinned pages and try short-term pinned pages during page >> reclaiming. But as you pointed out, vmsplice() doesn't use FOLL_PIN. >> So if vmsplice() is expected to pin pages for long time, and we have no >> way to detect it, then we should keep can_split_folio() in the original >> code. >> >> Copying more people who have worked on long-term pinning for comments. > > Checked the discussion in the following thread, > > https://lore.kernel.org/lkml/CA+CK2bBffHBxjmb9jmSKacm0fJMinyt3Nhk8Nx6iudcQSj80_w@mail.gmail.com/ > > It seems that from the practical point of view, folio_maybe_dma_pinned() > can identify most long-term pinned pages that may block memory hot- > remove or CMA allocation. Although as David pointed out, some pages may > still be GUPed for long time (e.g. via vmsplice) even if > !folio_maybe_dma_pinned(). > > But from another point of view, can_split_huge_page() is cheap and THP > swapout is expensive (swap space, disk IO, and hard to be recovered), so > it may be better to keep can_split_huge_page() in shink_page_list(). Many thanks for your explanation. Looks convincing for me. Is it worth a comment about the above stuff? Anyway, will drop this patch. Thanks! > > Best Regards, > Huang, Ying > >> >>> can_split_folio() is more precise then folio_maybe_dma_pinned(), but >>> both are racy as long as the page is still mapped. >>> >>> >> > > > . >