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 bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (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 EE2ADC3DA78 for ; Tue, 17 Jan 2023 21:56:02 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20210309; h=Sender: Content-Transfer-Encoding:Content-Type:List-Subscribe:List-Help:List-Post: List-Archive:List-Unsubscribe:List-Id:In-Reply-To:MIME-Version:References: Message-ID:Subject:Cc:To:From:Date:Reply-To:Content-ID:Content-Description: Resent-Date:Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID: List-Owner; bh=HfDZ/r6fxXmx7I24fR1Axjb76Yeu5XRamy1qnmB/hcs=; b=ankLXgAG7vdxW7 KTYj2WpWEqrxDhnFNZAJDgi4MwlzNhH6/jRMhPSnuJR4XxQlAJYjYUlwmQNNmHsgS+x3/nljpMV7/ 3x0m+0ib8BFMDd6XjJcW6EwfbTeOMyBavA6BcHx7wVdmzhyoUNC03OxELslBKErzV0l2GIKU3mCER tu4/WoR6FK7PPoD1lek0PCOlnIH3XkireBEI20Tb+cskKMFmMLh/MW0ZQfS0TpJ5SXQksPGuf6Xrp KWIMvolmT/5omJ5BYhOBO512bTEjUg5hQ3gRJArwAmT8vdu16Z7dkYO1kzFG0x4BnSAB8WZr5FcV2 k9YxNwboH9Qy43f3g4VQ==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.94.2 #2 (Red Hat Linux)) id 1pHtuy-00G2hZ-Kz; Tue, 17 Jan 2023 21:54:56 +0000 Received: from casper.infradead.org ([2001:8b0:10b:1236::1]) by bombadil.infradead.org with esmtps (Exim 4.94.2 #2 (Red Hat Linux)) id 1pHtuw-00G2hP-Vh for linux-arm-kernel@bombadil.infradead.org; Tue, 17 Jan 2023 21:54:54 +0000 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 Cc: Michal Hocko , akpm@linux-foundation.org, michel@lespinasse.org, jglisse@google.com, vbabka@suse.cz, hannes@cmpxchg.org, mgorman@techsingularity.net, dave@stgolabs.net, liam.howlett@oracle.com, peterz@infradead.org, ldufour@linux.ibm.com, laurent.dufour@fr.ibm.com, paulmck@kernel.org, luto@kernel.org, songliubraving@fb.com, peterx@redhat.com, david@redhat.com, dhowells@redhat.com, hughd@google.com, bigeasy@linutronix.de, kent.overstreet@linux.dev, punit.agrawal@bytedance.com, lstoakes@gmail.com, peterjung1337@gmail.com, rientjes@google.com, axelrasmussen@google.com, joelaf@google.com, minchan@google.com, jannh@google.com, shakeelb@google.com, tatashin@google.com, edumazet@google.com, gthelen@google.com, gurua@google.com, arjunroy@google.com, soheil@google.com, hughlynch@google.com, leewalsh@google.com, posk@google.com, linux-mm@kvack.org, linux-arm-kernel@lists.infradead.org, linuxppc-dev@lists.ozlabs.org, x86@kernel.org, linux-kernel@vger.kernel.org, kernel-team@android.com 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-Disposition: inline In-Reply-To: X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org 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 _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel