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[2003:cb:c708:2e00:4009:47dc:d0e5:dcd2]) by smtp.gmail.com with ESMTPSA id b6-20020a5d6346000000b0022063e5228bsm19597324wrw.93.2022.08.11.09.24.59 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 11 Aug 2022 09:24:59 -0700 (PDT) Message-ID: <2cb95372-f7dd-61f9-9d2a-461390fb0907@redhat.com> Date: Thu, 11 Aug 2022 18:24:59 +0200 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.11.0 Subject: Re: [PATCH v2 2/2] mm/hugetlb: support write-faults in shared mappings To: Peter Xu Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, stable@vger.kernel.org References: <20220811103435.188481-1-david@redhat.com> <20220811103435.188481-3-david@redhat.com> From: David Hildenbrand Organization: Red Hat In-Reply-To: X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1660235103; a=rsa-sha256; cv=none; b=CVq+Dv818QZ95Pc1KieWjUeK8mCLo4d3LEI0II+M4xGGzMfzovHK+GuO1jnPR8J9DqYBn6 TL7GE9T8NjuoYygArfEKrwVGZmc/ZJh9P4W/IVegVHyiYaffHg33WlLM3RwTL+Qix4XCgf Pu0Dgq7i4q5XiQle/kxWvzvC3X+qeao= ARC-Authentication-Results: i=1; imf09.hostedemail.com; dkim=pass header.d=redhat.com header.s=mimecast20190719 header.b=gONzUm84; spf=pass (imf09.hostedemail.com: domain of david@redhat.com designates 170.10.129.124 as permitted sender) smtp.mailfrom=david@redhat.com; dmarc=pass (policy=none) header.from=redhat.com ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1660235103; 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:content-transfer-encoding: in-reply-to:in-reply-to:references:references:dkim-signature; bh=UjYkrJ0NaTJji01ZcUTXu/HSgTuMTSkVyUx2biIqUJU=; b=C9aDhjkm6zu8PVZnf4D5LblX3IHyl9VJg7WoUXNKHW/EAyI10oOhk2cNy2nwc3hku+Mms+ IVMQIOxgpUJoxglka4ScEaURHnVNheU1iMlUaGMCtBb78WxQPW2AMxUNlgEtANvrOfaUjv O90o3LHC6o0XS9Lzznua2OIschHQw0A= X-Rspam-User: X-Stat-Signature: ikiwpafypci7dagyoabp1db4yto6ocam X-Rspamd-Queue-Id: 2D877140197 Authentication-Results: imf09.hostedemail.com; dkim=pass header.d=redhat.com header.s=mimecast20190719 header.b=gONzUm84; spf=pass (imf09.hostedemail.com: domain of david@redhat.com designates 170.10.129.124 as permitted sender) smtp.mailfrom=david@redhat.com; dmarc=pass (policy=none) header.from=redhat.com X-Rspamd-Server: rspam01 X-HE-Tag: 1660235103-873870 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 11.08.22 15:59, Peter Xu wrote: > On Thu, Aug 11, 2022 at 12:34:35PM +0200, David Hildenbrand wrote: >> Reason is that uffd-wp doesn't clear the uffd-wp PTE bit when >> unregistering and consequently keeps the PTE writeprotected. Reason for >> this is to avoid the additional overhead when unregistering. Note >> that this is the case also for !hugetlb and that we will end up with >> writable PTEs that still have the uffd-wp PTE bit set once we return >> from hugetlb_wp(). I'm not touching the uffd-wp PTE bit for now, because it >> seems to be a generic thing -- wp_page_reuse() also doesn't clear it. > > This may justify that lazy reset of ptes may not really be a good idea, > including anonymous. I'm indeed not aware of any app that do frequent > reg/unreg at least. Yeah. QEMU snapshots come to mind, but I guess the reg/unreg overhead is the smallest issue. > > I'll prepare a patch to change it from uffd side too. > > Thanks again for finding this problem. YW! -- Thanks, David / dhildenb