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[70.31.27.79]) by smtp.gmail.com with ESMTPSA id t20-20020a05620a451400b006ceb933a9fesm3408375qkp.81.2022.11.18.15.14.20 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 18 Nov 2022 15:14:21 -0800 (PST) Date: Fri, 18 Nov 2022 18:14:19 -0500 From: Peter Xu To: Muhammad Usama Anjum Cc: David Hildenbrand , Nadav Amit , Andrea Arcangeli , Andrew Morton , linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: Re: [PATCH v4 1/3] mm/mprotect: Fix soft-dirty check in can_change_pte_writable() Message-ID: References: <20220725142048.30450-1-peterx@redhat.com> <20220725142048.30450-2-peterx@redhat.com> MIME-Version: 1.0 In-Reply-To: X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Disposition: inline ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1668813265; a=rsa-sha256; cv=none; b=xU042TwnYPVK6Nw2QC7ZcoMj9RcaPNE68x/wpiHoIK0wuL9zn4PTM0CV7AZFwuj1rlWiY1 In/0yL43gSq46rfKtZPitKMGQRKhn+m+9EZWR3zUX2I0O1RI2jUQD0w8/EnWd/Zwylk5nJ k6RmCztQmdxH7bcRLIpiQ8fko1elJtk= ARC-Authentication-Results: i=1; imf11.hostedemail.com; dkim=pass header.d=redhat.com header.s=mimecast20190719 header.b="HoL/SSNh"; dmarc=pass (policy=none) header.from=redhat.com; spf=pass (imf11.hostedemail.com: domain of peterx@redhat.com designates 170.10.133.124 as permitted sender) smtp.mailfrom=peterx@redhat.com ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1668813265; h=from:from:sender:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type:content-transfer-encoding: in-reply-to:in-reply-to:references:references:dkim-signature; bh=l2sD/7nQDcm641haY/C3yaUw5XrSrZUByds8laMM8v4=; b=8QVkhEhLfbCZnehBW7mSfdHF7s0J2VPGT6HLJYAhen4wE0EM2N27krfkAa2b85FdOhTHtI zrY/hJp0Jhyvh1cFHMUeepUR7qwI7eGlSQsolcRkN23UYu5ARE5qsUrMQ/bQkxnZsLcusj 0SPdZ67sNcbqqi8AwGsWuwNbd30QrM0= X-Stat-Signature: 8bdubce996x9hqou3y9ego3jiekhqud9 X-Rspamd-Queue-Id: 9F8AF40009 Authentication-Results: imf11.hostedemail.com; dkim=pass header.d=redhat.com header.s=mimecast20190719 header.b="HoL/SSNh"; dmarc=pass (policy=none) header.from=redhat.com; spf=pass (imf11.hostedemail.com: domain of peterx@redhat.com designates 170.10.133.124 as permitted sender) smtp.mailfrom=peterx@redhat.com X-Rspam-User: X-Rspamd-Server: rspam12 X-HE-Tag: 1668813265-665216 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Sat, Nov 19, 2022 at 01:16:26AM +0500, Muhammad Usama Anjum wrote: > Hi Peter and David, Hi, Muhammad, > > On 7/25/22 7:20 PM, Peter Xu wrote: > > The check wanted to make sure when soft-dirty tracking is enabled we won't > > grant write bit by accident, as a page fault is needed for dirty tracking. > > The intention is correct but we didn't check it right because VM_SOFTDIRTY > > set actually means soft-dirty tracking disabled. Fix it. > [...] > > +static inline bool vma_soft_dirty_enabled(struct vm_area_struct *vma) > > +{ > > + /* > > + * NOTE: we must check this before VM_SOFTDIRTY on soft-dirty > > + * enablements, because when without soft-dirty being compiled in, > > + * VM_SOFTDIRTY is defined as 0x0, then !(vm_flags & VM_SOFTDIRTY) > > + * will be constantly true. > > + */ > > + if (!IS_ENABLED(CONFIG_MEM_SOFT_DIRTY)) > > + return false; > > + > > + /* > > + * Soft-dirty is kind of special: its tracking is enabled when the > > + * vma flags not set. > > + */ > > + return !(vma->vm_flags & VM_SOFTDIRTY); > > +} > I'm sorry. I'm unable to understand the inversion here. > > its tracking is enabled when the vma flags not set. > VM_SOFTDIRTY is set on the VMA when new VMA is allocated to mark is > soft-dirty. When we write to clear_refs to clear soft-dirty bit, > VM_SOFTDIRTY is cleared from the VMA as well. Then why do you say tracking > is enabled when the vma flags not set? Because only when 4>clear_refs happens would VM_SOFTDIRTY be cleared, and only until then the real tracking starts (by removing write bits on ptes). > I'm missing some obvious thing. Maybe the meaning of tracking is to see > if VM_SOFTDIRTY needs to be set. If VM_SOFTDIRTY is already set, tracking > isn't needed. Can you give an example here? If VM_SOFTDIRTY is set, pagemap will treat all pages as soft-dirty, please see pagemap_pmd_range(): if (vma->vm_flags & VM_SOFTDIRTY) flags |= PM_SOFT_DIRTY; So fundamentally it reports nothing useful when VM_SOFTDIRTY set. That's also why we need the clear_refs first before we can have anything useful. Feel free to reference to the doc page (admin-guide/mm/soft-dirty.rst): ---8<--- The soft-dirty is a bit on a PTE which helps to track which pages a task writes to. In order to do this tracking one should 1. Clear soft-dirty bits from the task's PTEs. This is done by writing "4" into the ``/proc/PID/clear_refs`` file of the task in question. 2. Wait some time. 3. Read soft-dirty bits from the PTEs. This is done by reading from the ``/proc/PID/pagemap``. The bit 55 of the 64-bit qword is the soft-dirty one. If set, the respective PTE was written to since step 1. ---8<--- The tracking starts at step 1, where is when the flag is cleared. Thanks, -- Peter Xu