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 lists.ozlabs.org (lists.ozlabs.org [112.213.38.117]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 47F0FC54E76 for ; Tue, 17 Jan 2023 21:56:29 +0000 (UTC) Received: from boromir.ozlabs.org (localhost [IPv6:::1]) by lists.ozlabs.org (Postfix) with ESMTP id 4NxN575LFGz3c46 for ; Wed, 18 Jan 2023 08:56:27 +1100 (AEDT) Authentication-Results: lists.ozlabs.org; dkim=fail reason="signature verification failed" (2048-bit key; secure) header.d=infradead.org header.i=@infradead.org header.a=rsa-sha256 header.s=casper.20170209 header.b=O6tZSHHz; dkim-atps=neutral Authentication-Results: lists.ozlabs.org; spf=none (no SPF record) smtp.mailfrom=infradead.org (client-ip=2001:8b0:10b:1236::1; helo=casper.infradead.org; envelope-from=willy@infradead.org; receiver=) Authentication-Results: lists.ozlabs.org; dkim=pass (2048-bit key; secure) header.d=infradead.org header.i=@infradead.org header.a=rsa-sha256 header.s=casper.20170209 header.b=O6tZSHHz; dkim-atps=neutral Received: from casper.infradead.org (casper.infradead.org [IPv6:2001:8b0:10b:1236::1]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by lists.ozlabs.org (Postfix) with ESMTPS id 4NxN452kBgz2yg5 for ; Wed, 18 Jan 2023 08:55:32 +1100 (AEDT) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=casper.20170209; h=In-Reply-To:Content-Type:MIME-Version: References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=dYK7QHEqGROj9cTFeHvfSuKosaNcqMrUb865g495bOk=; b=O6tZSHHzGKJZX1iAYHUlbpNdcq ZxrxvmTmmsbqr936EiEWOWMMqob8GgiHmdccZoQjFy2M7JSHlbZzbvs2TchncI+j0k3viYTmseIPC SRjOwRtExn9hUARvpUyEETZYLIXkShGyDfwMwJ/WYav3L6y7Iyc00H+U3nCuYm8W5n4LDo5BjuZ9p +4aRxAyoACCM8sQessBpBPk+zQMyvZ5Hh/ve5p02fFVP7yXt2nK4sqO5wiKupykfS5P04QW3XJyeU 2jtbz1crNDA/cVwMfi6Bbdl8kHWAt1s79upbuw/UbeV07ohd0sPjx5OZI3OnS4vNsVmqG228akiwH AVrAhxbQ==; Received: from willy by casper.infradead.org with local (Exim 4.94.2 #2 (Red Hat Linux)) id 1pHtv0-00A4f1-Fg; Tue, 17 Jan 2023 21:54:58 +0000 Date: Tue, 17 Jan 2023 21:54:58 +0000 From: Matthew Wilcox To: Suren Baghdasaryan Subject: Re: [PATCH 12/41] mm: add per-VMA lock and helper functions to control it Message-ID: References: <20230109205336.3665937-1-surenb@google.com> <20230109205336.3665937-13-surenb@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-BeenThere: linuxppc-dev@lists.ozlabs.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: michel@lespinasse.org, joelaf@google.com, songliubraving@fb.com, Michal Hocko , leewalsh@google.com, david@redhat.com, peterz@infradead.org, bigeasy@linutronix.de, peterx@redhat.com, dhowells@redhat.com, linux-mm@kvack.org, edumazet@google.com, jglisse@google.com, punit.agrawal@bytedance.com, arjunroy@google.com, dave@stgolabs.net, minchan@google.com, x86@kernel.org, hughd@google.com, gurua@google.com, laurent.dufour@fr.ibm.com, linux-arm-kernel@lists.infradead.org, rientjes@google.com, axelrasmussen@google.com, kernel-team@android.com, soheil@google.com, paulmck@kernel.org, jannh@google.com, liam.howlett@oracle.com, shakeelb@google.com, luto@kernel.org, gthelen@google.com, ldufour@linux.ibm.com, vbabka@suse.cz, posk@google.com, lstoakes@gmail.com, peterjung1337@gmail.com, linuxppc-dev@lists.ozlabs.org, kent.overstreet@linux.dev, hughlynch@google.com, linux-kernel@vger.kernel.org, hannes@cmpxchg.org, akpm@linux-foundation.org, tatashin@google.com, mgorman@techsin gularity.net Errors-To: linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org Sender: "Linuxppc-dev" On Tue, Jan 17, 2023 at 01:21:47PM -0800, Suren Baghdasaryan wrote: > On Tue, Jan 17, 2023 at 7:12 AM Michal Hocko wrote: > > > > On Tue 17-01-23 16:04:26, Michal Hocko wrote: > > > On Mon 09-01-23 12:53:07, Suren Baghdasaryan wrote: > > > > Introduce a per-VMA rw_semaphore to be used during page fault handling > > > > instead of mmap_lock. Because there are cases when multiple VMAs need > > > > to be exclusively locked during VMA tree modifications, instead of the > > > > usual lock/unlock patter we mark a VMA as locked by taking per-VMA lock > > > > exclusively and setting vma->lock_seq to the current mm->lock_seq. When > > > > mmap_write_lock holder is done with all modifications and drops mmap_lock, > > > > it will increment mm->lock_seq, effectively unlocking all VMAs marked as > > > > locked. > > > > > > I have to say I was struggling a bit with the above and only understood > > > what you mean by reading the patch several times. I would phrase it like > > > this (feel free to use if you consider this to be an improvement). > > > > > > Introduce a per-VMA rw_semaphore. The lock implementation relies on a > > > per-vma and per-mm sequence counters to note exclusive locking: > > > - read lock - (implemented by vma_read_trylock) requires the the > > > vma (vm_lock_seq) and mm (mm_lock_seq) sequence counters to > > > differ. If they match then there must be a vma exclusive lock > > > held somewhere. > > > - read unlock - (implemented by vma_read_unlock) is a trivial > > > vma->lock unlock. > > > - write lock - (vma_write_lock) requires the mmap_lock to be > > > held exclusively and the current mm counter is noted to the vma > > > side. This will allow multiple vmas to be locked under a single > > > mmap_lock write lock (e.g. during vma merging). The vma counter > > > is modified under exclusive vma lock. > > > > Didn't realize one more thing. > > Unlike standard write lock this implementation allows to be > > called multiple times under a single mmap_lock. In a sense > > it is more of mark_vma_potentially_modified than a lock. > > In the RFC it was called vma_mark_locked() originally and renames were > discussed in the email thread ending here: > https://lore.kernel.org/all/621612d7-c537-3971-9520-a3dec7b43cb4@suse.cz/. > If other names are preferable I'm open to changing them. I don't want to bikeshed this, but rather than locking it seems to be more: vma_start_read() vma_end_read() vma_start_write() vma_end_write() vma_downgrade_write() ... and that these are _implemented_ with locks (in part) is an implementation detail? Would that reduce people's confusion? > > > > > - write unlock - (vma_write_unlock_mm) is a batch release of all > > > vma locks held. It doesn't pair with a specific > > > vma_write_lock! It is done before exclusive mmap_lock is > > > released by incrementing mm sequence counter (mm_lock_seq). > > > - write downgrade - if the mmap_lock is downgraded to the read > > > lock all vma write locks are released as well (effectivelly > > > same as write unlock). > > -- > > Michal Hocko > > SUSE Labs