From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from madrid.collaboradmins.com (madrid.collaboradmins.com [46.235.227.194]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 2746442055; Mon, 29 Apr 2024 13:28:26 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=46.235.227.194 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1714397308; cv=none; b=QRDGwS9kTC6XPVAksyRgHLb0BN9CRQOgJQa0cYW2HKOmwlUUw8XaDozEhegwyg+FA9LLNgCzxVOBM1kSsxz88i9tY+PrienmLEcwdU5O0UyUqeEs0n2nkibn/MEdsMOhXY+gsV2hzJaxukw1/1IBdK1y6ZtCX8y+kEyCrCIU5J8= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1714397308; c=relaxed/simple; bh=w3I0KB3Wx4VUBmPLi5oXkCJcbKLadPaoVRj5i8XU6Lg=; h=Message-ID:Date:MIME-Version:Cc:Subject:To:References:From: In-Reply-To:Content-Type; b=f3gU/e+rVXWSIMrR7ep638U26Qj/eqZNE9why9TKjdkIbv3DM8GVFLS3WfTWb6v5w8JdXGjT4+D3F36yaL+B2ScAqch3vNnkfTV0KBm00QtA9LdS2EjWn1tiq2L8N4QTFedt5yfCFdAw0WV7WKIWLL/lX+i3bu5Jr6okGhjzbSg= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=collabora.com; spf=pass smtp.mailfrom=collabora.com; dkim=pass (2048-bit key) header.d=collabora.com header.i=@collabora.com header.b=iZOgrpSG; arc=none smtp.client-ip=46.235.227.194 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=collabora.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=collabora.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=collabora.com header.i=@collabora.com header.b="iZOgrpSG" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=collabora.com; s=mail; t=1714397305; bh=w3I0KB3Wx4VUBmPLi5oXkCJcbKLadPaoVRj5i8XU6Lg=; h=Date:Cc:Subject:To:References:From:In-Reply-To:From; b=iZOgrpSGYFamkt2yD9BicbFgxnhdSs/yuGP6l/j6ZBNgNUmhGWlMTORX2defVI0se jy5V2yW9njRyq7xaxVNNw0npZa1qoYBH4yRy9b7R3jOLF4YdA/FF0cJGez2wTWN7Yo W4Pv02X9j2RBS8a2oeIozt+WO4ZoODUCbFUVh79pLd8k9H+kgXxbhT4XdLmC4rD0wW f1q9gXNpO8QSMOnDeWUoQ5v/OwkJbaZx/qZv/pAQ1wmNMk3XvA60wzNQ+RHhIHPa8t 6FsYuVP8AaaSBzbigltgMMJb6kE4GVvDTQ2OL67bLICmDLvdj8B+nT8PrcC89q3pa0 MBxpQ/IiEGDUQ== Received: from [10.193.1.1] (broslavsky.collaboradmins.com [68.183.210.73]) (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 madrid.collaboradmins.com (Postfix) with ESMTPSA id A333A3780C22; Mon, 29 Apr 2024 13:28:23 +0000 (UTC) Message-ID: <700ae50c-2fd6-4bcc-8840-f426558b847f@collabora.com> Date: Mon, 29 Apr 2024 18:28:53 +0500 Precedence: bulk X-Mailing-List: linux-fsdevel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Cc: Muhammad Usama Anjum , linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org Subject: Re: [PATCH v1] fs/proc/task_mmu: Fix loss of young/dirty bits during pagemap scan To: Ryan Roberts , Andrew Morton , David Hildenbrand , Peter Xu References: <20240429114017.182570-1-ryan.roberts@arm.com> Content-Language: en-US From: Muhammad Usama Anjum In-Reply-To: <20240429114017.182570-1-ryan.roberts@arm.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Thanks! On 4/29/24 4:40 PM, Ryan Roberts wrote: > make_uffd_wp_pte() was previously doing: > > pte = ptep_get(ptep); > ptep_modify_prot_start(ptep); > pte = pte_mkuffd_wp(pte); > ptep_modify_prot_commit(ptep, pte); > > But if another thread accessed or dirtied the pte between the first 2 > calls, this could lead to loss of that information. Since > ptep_modify_prot_start() gets and clears atomically, the following is > the correct pattern and prevents any possible race. Any access after the > first call would see an invalid pte and cause a fault: > > pte = ptep_modify_prot_start(ptep); > pte = pte_mkuffd_wp(pte); > ptep_modify_prot_commit(ptep, pte); > > Fixes: 52526ca7fdb9 ("fs/proc/task_mmu: implement IOCTL to get and optionally clear info about PTEs") > Signed-off-by: Ryan Roberts Reviewed-by: Muhammad Usama Anjum Tested-by: Muhammad Usama Anjum > --- > fs/proc/task_mmu.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c > index 23fbab954c20..af4bc1da0c01 100644 > --- a/fs/proc/task_mmu.c > +++ b/fs/proc/task_mmu.c > @@ -1825,7 +1825,7 @@ static void make_uffd_wp_pte(struct vm_area_struct *vma, > pte_t old_pte; > > old_pte = ptep_modify_prot_start(vma, addr, pte); > - ptent = pte_mkuffd_wp(ptent); > + ptent = pte_mkuffd_wp(old_pte); > ptep_modify_prot_commit(vma, addr, pte, old_pte, ptent); > } else if (is_swap_pte(ptent)) { > ptent = pte_swp_mkuffd_wp(ptent); > -- > 2.25.1 > -- BR, Muhammad Usama Anjum