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[2003:cb:c702:6300:c44f:789a:59b5:91e9]) by smtp.gmail.com with ESMTPSA id m16-20020adfe950000000b0021d70a871cbsm10008073wrn.32.2022.07.08.04.47.47 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 08 Jul 2022 04:47:48 -0700 (PDT) Message-ID: <0864a811-53c8-a87b-a32d-d6f4c7945caa@redhat.com> Date: Fri, 8 Jul 2022 13:47:46 +0200 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.11.0 To: Andrew Morton , Khalid Aziz Cc: willy@infradead.org, aneesh.kumar@linux.ibm.com, arnd@arndb.de, 21cnbao@gmail.com, corbet@lwn.net, dave.hansen@linux.intel.com, ebiederm@xmission.com, hagen@jauu.net, jack@suse.cz, keescook@chromium.org, kirill@shutemov.name, kucharsk@gmail.com, linkinjeon@kernel.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, longpeng2@huawei.com, luto@kernel.org, markhemm@googlemail.com, pcc@google.com, rppt@kernel.org, sieberf@amazon.com, sjpark@amazon.de, surenb@google.com, tst@schoebel-theuer.de, yzaikin@google.com References: <20220701212403.77ab8139b6e1aca87fae119e@linux-foundation.org> From: David Hildenbrand Organization: Red Hat Subject: Re: [PATCH v2 0/9] Add support for shared PTEs across processes In-Reply-To: <20220701212403.77ab8139b6e1aca87fae119e@linux-foundation.org> 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=1657280873; a=rsa-sha256; cv=none; b=AYI4pfFAjViSty79MOiOZQs9+jRd1voHCyel4RljH667ipZc5KVngrqe3Xep6XZSkeL5wV Q2KbbCxHdUd4i7eEw5iDZEnQG9dR3v6oa2qhJE/pVwzgkqrDN3S50OmAbHMh5FlC3mHrJr 4n8evpAnB3P6qpzBBML3xi8ix1rkPx4= ARC-Authentication-Results: i=1; imf30.hostedemail.com; dkim=pass header.d=redhat.com header.s=mimecast20190719 header.b=ZWE8UQw1; spf=none (imf30.hostedemail.com: domain of david@redhat.com has no SPF policy when checking 170.10.129.124) 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=1657280873; 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=4qkbfqfqgKZl4vr074ljOBTMMH/P10CPWSxYVWQ0VaQ=; b=MAConY7eZqsGJQVDVfM9RLBNWzzjH7Ng4LN82z9uaHA0Ysw6GdF+8n9/7H+OnrHo/9459C p8R039lTKwSH0gEagT4nFLQspvDH6yPZQAdlVbGfuE+EPnaT62i/qv7mUu91O3wISR16+V 8go0uY9cF4W4+xeuPV7cjFKPGUBL9lc= X-Stat-Signature: gq9x6jho4o6j6j9qbyju19fmkc83jaa4 X-Rspamd-Queue-Id: AC12880057 Authentication-Results: imf30.hostedemail.com; dkim=pass header.d=redhat.com header.s=mimecast20190719 header.b=ZWE8UQw1; spf=none (imf30.hostedemail.com: domain of david@redhat.com has no SPF policy when checking 170.10.129.124) smtp.mailfrom=david@redhat.com; dmarc=pass (policy=none) header.from=redhat.com X-Rspam-User: X-Rspamd-Server: rspam10 X-HE-Tag: 1657280873-478117 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 02.07.22 06:24, Andrew Morton wrote: > On Wed, 29 Jun 2022 16:53:51 -0600 Khalid Aziz wrote: > >> This patch series implements a mechanism in kernel to allow >> userspace processes to opt into sharing PTEs. It adds a new >> in-memory filesystem - msharefs. > > Dumb question: why do we need a new filesystem for this? Is it not > feasible to permit PTE sharing for mmaps of tmpfs/xfs/ext4/etc files? > IIRC, the general opinion at LSF/MM was that this approach at hand is makes people nervous and I at least am not convinced that we really want to have this upstream. What's *completely* missing from the cover letter are the dirty details: "Actual data is mmap'ed using anonymous pages, ext4/xfs/btfrfs/etc files.". Gah. As raised already, "anonymous pages" makes me shiver. (To me, what I read, this looks like an RFC to me, yet I see "v2". But I am a little confused why most of the feedback at LSF/MM seems to be ignored and people are moving forward with this approach. But maybe my memory is wrong.) Please, let's look into more generic page table sharing just like hugetlb already provides to some degree. And if we need additional alignment requirements to share multiple page table levels, then let's look into that as well for special users. -- Thanks, David / dhildenb