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 9EEDCC6FD1D for ; Fri, 7 Apr 2023 10:15:04 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231239AbjDGKPD (ORCPT ); Fri, 7 Apr 2023 06:15:03 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:46828 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231204AbjDGKPB (ORCPT ); Fri, 7 Apr 2023 06:15:01 -0400 Received: from madras.collabora.co.uk (madras.collabora.co.uk [IPv6:2a00:1098:0:82:1000:25:2eeb:e5ab]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 9FEFA8A5B; Fri, 7 Apr 2023 03:15:00 -0700 (PDT) Received: from [192.168.10.39] (unknown [119.155.57.40]) (using TLSv1.3 with cipher TLS_AES_128_GCM_SHA256 (128/128 bits) key-exchange X25519 server-signature RSA-PSS (4096 bits) server-digest SHA256) (No client certificate requested) (Authenticated sender: usama.anjum) by madras.collabora.co.uk (Postfix) with ESMTPSA id 637A46603131; Fri, 7 Apr 2023 11:14:52 +0100 (BST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=collabora.com; s=mail; t=1680862498; bh=9WP6pUpBkOjkcvFCjLQ/X0oqCxKf/YO1tbGg0IwaD8g=; h=Date:Cc:Subject:To:References:From:In-Reply-To:From; b=cXZpGyfbBDqWqi9rMvJzyzWzubRZSu9ZlI+W4zmtflNBkvxlViQd2BU5RAr7Rb2Xb zgsym/vALXf5K1nMX2MfNXL6VmHmmDgVMTocbmjp2LTDUH4Xz43C+q5YYfHLYHz7ho 9viO3eJkjQUYJvB8MB6IMDAWq0Xqn8UXep3s6yZTtXm1X9FbaJW0B5blLUvnpy6Zge aU5fIqi3eBVWRKrouL0yLn7dgictkTDyK0VYY1yyUVZaH52po+CYg5e0Mgs1M3PWmy wByDH8kZ/Qwiet4sxyGVYesTr62q6iKAr98uEPY6dzO4DUJJrmruewJ9traJ8MEioM X06VRpYxIA9LQ== Message-ID: Date: Fri, 7 Apr 2023 15:14:47 +0500 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.9.0 Cc: Muhammad Usama Anjum , Peter Xu , David Hildenbrand , Andrew Morton , Andrei Vagin , Danylo Mocherniuk , Paul Gofman , Cyrill Gorcunov , Mike Rapoport , Nadav Amit , Alexander Viro , Shuah Khan , Christian Brauner , Yang Shi , Vlastimil Babka , "Liam R . Howlett" , Yun Zhou , Suren Baghdasaryan , Alex Sierra , Matthew Wilcox , Pasha Tatashin , Axel Rasmussen , "Gustavo A . R . Silva" , Dan Williams , linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, linux-kselftest@vger.kernel.org, Greg KH , kernel@collabora.com Subject: Re: [PATCH v12 2/5] fs/proc/task_mmu: Implement IOCTL to get and optionally clear info about PTEs To: =?UTF-8?B?TWljaGHFgiBNaXJvc8WCYXc=?= References: <20230406074005.1784728-1-usama.anjum@collabora.com> <20230406074005.1784728-3-usama.anjum@collabora.com> Content-Language: en-US From: Muhammad Usama Anjum In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kselftest@vger.kernel.org On 4/7/23 3:04 PM, Michał Mirosław wrote: > On Fri, 7 Apr 2023 at 11:35, Muhammad Usama Anjum > wrote: >> On 4/7/23 12:23 PM, Michał Mirosław wrote: >>> On Thu, 6 Apr 2023 at 23:12, Muhammad Usama Anjum >>> wrote: >>>> On 4/7/23 1:12 AM, Michał Mirosław wrote: >>>>> On Thu, 6 Apr 2023 at 09:40, Muhammad Usama Anjum >>>>> wrote: >>>>> [...] >>>>>> --- a/fs/proc/task_mmu.c >>>>>> +++ b/fs/proc/task_mmu.c >>>>> [...] >>>>>> +static int pagemap_scan_pmd_entry(pmd_t *pmd, unsigned long start, >>>>>> + unsigned long end, struct mm_walk *walk) >>>>>> +{ >>> [...] >>>>>> +#ifdef CONFIG_TRANSPARENT_HUGEPAGE >>>>>> + ptl = pmd_trans_huge_lock(pmd, vma); >>>>>> + if (ptl) { >>>>> [...] >>>>>> + return ret; >>>>>> + } >>>>>> +process_smaller_pages: >>>>>> + if (pmd_trans_unstable(pmd)) >>>>>> + return 0; >>>>> >>>>> Why pmd_trans_unstable() is needed here and not only after split_huge_pmd()? >>>> I'm not entirely sure. But the idea is if THP is unstable, we should >>>> return. As it doesn't seem like after splitting THP can be unstable, we >>>> should not check it. Do you agree with the following? >>> >>> The description of pmd_trans_unstable() [1] seems to indicate that it >>> is needed only after split_huge_pmd(). >>> >>> [1] https://elixir.bootlin.com/linux/v6.3-rc5/source/include/linux/pgtable.h#L1394 >> Sorry, yeah pmd_trans_unstable() is need after split. But it is also needed >> in normal case when ptl is NULL to rule out the case if pmd is unstable >> before performing operation on normal pages: >> >> ptl = pmd_trans_huge_lock(pmd, vma); >> if (ptl) { >> ... >> } >> if (pmd_trans_unstable(pmd)) >> return 0; >> >> This file has usage examples of pmd_trans_unstable(): >> >> https://elixir.bootlin.com/linux/v6.3-rc5/source/fs/proc/task_mmu.c#L634 >> https://elixir.bootlin.com/linux/v6.3-rc5/source/fs/proc/task_mmu.c#L1195 >> https://elixir.bootlin.com/linux/v6.3-rc5/source/fs/proc/task_mmu.c#L1543 >> https://elixir.bootlin.com/linux/v6.3-rc5/source/fs/proc/task_mmu.c#L1887 >> >> So we are good with what we have in this patch. > > Shouldn't we signal ACTION_AGAIN then in order to call .pte_hole? I'm not sure. I've not done research on it if we need to signal ACTION_AGAIN as this function pagemap_scan_pmd_entry() mimics how pagemap_pmd_range() handles reads to the pagemap file. pagemap_pmd_range() isn't doing anything if pmd is unstable. Hence we also not doing anything. > > Best Regards > Michał Mirosław -- BR, Muhammad Usama Anjum