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 vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 32BC8C38159 for ; Tue, 17 Jan 2023 23:35:20 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230041AbjAQXfP (ORCPT ); Tue, 17 Jan 2023 18:35:15 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:59772 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230053AbjAQXeb (ORCPT ); Tue, 17 Jan 2023 18:34:31 -0500 Received: from mail-yw1-x1131.google.com (mail-yw1-x1131.google.com [IPv6:2607:f8b0:4864:20::1131]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 69A1A22782 for ; Tue, 17 Jan 2023 13:22:00 -0800 (PST) Received: by mail-yw1-x1131.google.com with SMTP id 00721157ae682-4d0f843c417so327708217b3.7 for ; Tue, 17 Jan 2023 13:22:00 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20210112; h=cc:to:subject:message-id:date:from:in-reply-to:references :mime-version:from:to:cc:subject:date:message-id:reply-to; bh=xtczcpnEdrb6Lo/I50tBUUcvRCWBMIoMko6b3VfQkh8=; b=AcMcOiO2z6ozFh44VpqFr7ezkcbEZnhi09Dbp343HQiIrdSXkgI50tblBtSXc4Qkbr Dm0aJ5OWtPIPe1vN3owVIfQugxfobInM08+GWcQC0Wl/sGleOffZOyKeiGBU66WnPH9X yGh7HPsaf3D37qff2nHLKSk9GB5gXj4/bJSoqoPygqkBfp0n5aCUJW1R2r1cGv48MYT9 AF0QYZ+3MvoaJhRFmZkp/7b87hU6GOckZwAwfWDK+vU6aqzT+gjcR5kQJuyq+Vt1cTTM SPO3L3JDVl95dXMuhHrtNaPJurOeiaEkl6ib7er/Q0Jv+k3LnRbtNLzrD3OvrfjKSDHz HteA== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=cc:to:subject:message-id:date:from:in-reply-to:references :mime-version:x-gm-message-state:from:to:cc:subject:date:message-id :reply-to; bh=xtczcpnEdrb6Lo/I50tBUUcvRCWBMIoMko6b3VfQkh8=; b=NYx4CjOABkOChRufeEVNXZEe1cUkPZCrIME8i3aNKSZXM4cNMSrby7HC/DBtkW2YFU g0jDEe8K45RMpqljwmSGfGUV/xgZM2O3kUP4Mr6h+sv889KjQqosS/7suR3/cj+iRVYr 8cl17C3aCMJhMCA0N7D46smTV+t9l1YGNEw+TBoow7k1az/ka3Bxk094qig2J9PH/Vfw av901gdBUdKYwpdrSYrzl+BjixdtA1fwozvAsLsWSJAbses4P24MVhQ+gxUSAOB7Z3xA c8InycSHWPQ6vPHLtYVsRigqFSFgPog4SDl7zE+zMKbdTGy4fRYE9kzO86EPHYD2Za8g HSwQ== X-Gm-Message-State: AFqh2kp/uPL85s8DcWU05IIRycUBMWh3MiK7H2Q64gupcOBvcmkb7WHp xMBc852ZAV8Oh/mpnTnRyIbX35ZCy9w1GoVOurqU5A== X-Google-Smtp-Source: AMrXdXtePqeFLTDxDaGD0SiAwIlsFIJGXklyX1m/MZjPOlYrdLeSS6NZFiXQQ8abRq5qEH0JCgU5ZQh27jLTDLKC0bU= X-Received: by 2002:a81:1d2:0:b0:433:f1c0:3f1c with SMTP id 201-20020a8101d2000000b00433f1c03f1cmr623569ywb.438.1673990519331; Tue, 17 Jan 2023 13:21:59 -0800 (PST) MIME-Version: 1.0 References: <20230109205336.3665937-1-surenb@google.com> <20230109205336.3665937-13-surenb@google.com> In-Reply-To: From: Suren Baghdasaryan Date: Tue, 17 Jan 2023 13:21:47 -0800 Message-ID: Subject: Re: [PATCH 12/41] mm: add per-VMA lock and helper functions to control it To: Michal Hocko Cc: akpm@linux-foundation.org, michel@lespinasse.org, jglisse@google.com, vbabka@suse.cz, hannes@cmpxchg.org, mgorman@techsingularity.net, dave@stgolabs.net, willy@infradead.org, 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 Content-Type: text/plain; charset="UTF-8" Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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. > > > - 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