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 C9F2BC77B6F for ; Fri, 7 Apr 2023 10:50:32 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S240607AbjDGKub (ORCPT ); Fri, 7 Apr 2023 06:50:31 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:41384 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231175AbjDGKu3 (ORCPT ); Fri, 7 Apr 2023 06:50:29 -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 CD7CB49DB; Fri, 7 Apr 2023 03:50:27 -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)) (No client certificate requested) (Authenticated sender: usama.anjum) by madras.collabora.co.uk (Postfix) with ESMTPSA id 88EE466031A4; Fri, 7 Apr 2023 11:50:19 +0100 (BST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=collabora.com; s=mail; t=1680864625; bh=P8yF/E8G1GvIUrwOi1mexCebBBHEtFanTu2WunsLvz4=; h=Date:Cc:Subject:To:References:From:In-Reply-To:From; b=jwsJ0V8HH/MiLKUqPJXwtPH6Cl83QV0xaT69QVX8eGPFiWvWj8onH2k5LGYcN4h8w XfloZVX9ALnP7RHZEPNb2uxErVmxGpCvgPU1ffnVok7cBtsgxlQ35pcDL2nAFMJi8P GwqeZprQyFMOK2RDiGC8gLvsUEiYqpZF/AFsPUvY5/2Gj95lm0KzBA1IbaJ7vXJxIu kxb21+y0GPZ3FQs4IastwAuTWZMp57lfLSQPC8yf/TE3oW12EX2SvQ8B3JpsSLmHhv qjWxUWksyd9RvWdjYnAT+MoupIMjYuGVrfL+QEJfXraDttUg/ZVa7FUi1pVrqXzUDR m0yoRpQKaxJ2g== Message-ID: <2992d40c-ddac-ed5f-ca80-8d3fc08b10e6@collabora.com> Date: Fri, 7 Apr 2023 15:50:14 +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 Content-Language: en-US To: =?UTF-8?B?TWljaGHFgiBNaXJvc8WCYXc=?= References: <20230406074005.1784728-1-usama.anjum@collabora.com> <20230406074005.1784728-3-usama.anjum@collabora.com> 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-fsdevel@vger.kernel.org On 4/7/23 3:21 PM, Michał Mirosław wrote: > On Fri, 7 Apr 2023 at 12:15, Muhammad Usama Anjum > wrote: >> 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. > > Doesn't this mean that if we scan a file-backed vma we would miss > non-present parts of the mapping in the output? I'm trying to mimic the same information through ioctl which is attained by reading the file. We'll only miss the unstable VMA here. I'm don't know about how often the PMD is unstable and its effects. > > Best Regards > Michał Mirosław -- BR, Muhammad Usama Anjum