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 068B0C3DA78 for ; Tue, 17 Jan 2023 04:35:04 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S235312AbjAQEe7 (ORCPT ); Mon, 16 Jan 2023 23:34:59 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:57974 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S235199AbjAQEeu (ORCPT ); Mon, 16 Jan 2023 23:34:50 -0500 Received: from mail-yw1-x1130.google.com (mail-yw1-x1130.google.com [IPv6:2607:f8b0:4864:20::1130]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 081082196F for ; Mon, 16 Jan 2023 20:34:49 -0800 (PST) Received: by mail-yw1-x1130.google.com with SMTP id 00721157ae682-4d59d518505so237880807b3.1 for ; Mon, 16 Jan 2023 20:34:48 -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=yRWBrMnmbVU6x7T0LPdTvz2nVvy3IZspLp0UQMF1irY=; b=hM38RCzSZziAG4zDZljPcNKfKjFLxHw4U/hoqCGnc2uhhpiLyfSuGpOTWJdWIMlxUv KjjhlBQmiQD8oPmw8rC7puYkrtFzSPrLW3kUpLnqfZC4c3aQzBYS/77dZgZ3jxcmGSFw GIuig9k6wQaOTlDLxqIH8XTIgBM6dLPj+spsFwsvjdG67Vdq/GY9waM9rX8RSrpCp0x3 8BaL/NOYp7Q0chsNamh7WaEtB2GnL40hJ1GgUHERDPj8PKO5UjxdqJIQNfLL/BykPZf1 B0tiiASa/wxaXtgB66A7Qm1/fEYloC8AH7VIbm0aHwYy1hzVUqvaTtSyE9Wgwz9LlOij 3bYA== 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=yRWBrMnmbVU6x7T0LPdTvz2nVvy3IZspLp0UQMF1irY=; b=wXPIxQoNNlaR6xVo1qCaHMW/fCRgrwfvTxcxLhADQQ+/opXVBTQrUCz77vfHl80oZr CXuYLClmqTjd++j+GnODdE5iTCXUAAZWvLiuX0PGaZH3yAYqanAn8ebkHY2QOLbmQQV2 DKTgKe4JSiQR8jvYuiECwx58wiU5u03x6sHRw2KJCeQoUeNL+veGUzIbkqRj7nSjHQrN X+dcqyCJLmi9mAgKbViREFfiYhJMeedHwwQ5JtNFBzZkToB2hkDCeH8pB5F+2sn6UATU cba6BSh+VbtV7LDPn1noxTS2QtXxPTjB1J7DbbDmGNJZjm4Xcc9R874Hy0FbSu6rE9rr DYiA== X-Gm-Message-State: AFqh2kofRbg96sNzuhXT8v+QOXjAGFUO0BPWePuJWsyaO8blU0qqKYLP BFfQ1NZoIvbK/hpREsRbdyTLZnsEnEmlfvHwUKaPbw== X-Google-Smtp-Source: AMrXdXseXKT6jNFxFMT1V3/l2fB00iDOPtEl/NQbThFwW0Hm+cjzFDqP3MAncylx3ySdnaL+hEiuL1zL4iLHm5r9qaI= X-Received: by 2002:a81:708c:0:b0:479:b6a1:d9a4 with SMTP id l134-20020a81708c000000b00479b6a1d9a4mr179033ywc.263.1673930087969; Mon, 16 Jan 2023 20:34:47 -0800 (PST) MIME-Version: 1.0 References: <20230109205336.3665937-1-surenb@google.com> <20230109205336.3665937-42-surenb@google.com> In-Reply-To: From: Suren Baghdasaryan Date: Mon, 16 Jan 2023 20:34:36 -0800 Message-ID: Subject: Re: [PATCH 41/41] mm: replace rw_semaphore with atomic_t in vma_lock To: Matthew Wilcox Cc: Hyeonggon Yoo <42.hyeyoo@gmail.com>, akpm@linux-foundation.org, michel@lespinasse.org, jglisse@google.com, mhocko@suse.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 Content-Type: text/plain; charset="UTF-8" Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Jan 16, 2023 at 8:14 PM Matthew Wilcox wrote: > > On Mon, Jan 16, 2023 at 11:14:38AM +0000, Hyeonggon Yoo wrote: > > > @@ -643,20 +647,28 @@ static inline void vma_write_lock(struct vm_area_struct *vma) > > > static inline bool vma_read_trylock(struct vm_area_struct *vma) > > > { > > > /* Check before locking. A race might cause false locked result. */ > > > - if (vma->vm_lock_seq == READ_ONCE(vma->vm_mm->mm_lock_seq)) > > > + if (vma->vm_lock->lock_seq == READ_ONCE(vma->vm_mm->mm_lock_seq)) > > > return false; > > > > > > - if (unlikely(down_read_trylock(&vma->vm_lock->lock) == 0)) > > > + if (unlikely(!atomic_inc_unless_negative(&vma->vm_lock->count))) > > > return false; > > > > > > + /* If atomic_t overflows, restore and fail to lock. */ > > > + if (unlikely(atomic_read(&vma->vm_lock->count) < 0)) { > > > + if (atomic_dec_and_test(&vma->vm_lock->count)) > > > + wake_up(&vma->vm_mm->vma_writer_wait); > > > + return false; > > > + } > > > + > > > /* > > > * Overflow might produce false locked result. > > > * False unlocked result is impossible because we modify and check > > > * vma->vm_lock_seq under vma->vm_lock protection and mm->mm_lock_seq > > > * modification invalidates all existing locks. > > > */ > > > - if (unlikely(vma->vm_lock_seq == READ_ONCE(vma->vm_mm->mm_lock_seq))) { > > > - up_read(&vma->vm_lock->lock); > > > + if (unlikely(vma->vm_lock->lock_seq == READ_ONCE(vma->vm_mm->mm_lock_seq))) { > > > + if (atomic_dec_and_test(&vma->vm_lock->count)) > > > + wake_up(&vma->vm_mm->vma_writer_wait); > > > return false; > > > } > > > > With this change readers can cause writers to starve. > > What about checking waitqueue_active() before or after increasing > > vma->vm_lock->count? > > I don't understand how readers can starve a writer. Readers do > atomic_inc_unless_negative() so a writer can always force readers > to fail. I think the point here was that if page faults keep occuring and they prevent vm_lock->count from reaching 0 then a writer will be blocked and there is no reader throttling mechanism (no max time that writer will be waiting).