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 32DB3C3DA78 for ; Tue, 17 Jan 2023 04:16:06 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S235553AbjAQEQA (ORCPT ); Mon, 16 Jan 2023 23:16:00 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:53750 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S235303AbjAQEPT (ORCPT ); Mon, 16 Jan 2023 23:15:19 -0500 Received: from casper.infradead.org (casper.infradead.org [IPv6:2001:8b0:10b:1236::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id D4DA72385A for ; Mon, 16 Jan 2023 20:15:16 -0800 (PST) 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=eeaHVXVVYLUZhPJsx3SPIZ13iIAgts/Qp1qngWaTzqA=; b=V1SsFbxr+Ofyz74nBDy89AeApK ITCGUjTbUYk0t0GnfGJvQhOi9bTVMlDqqYPb8ZKyHGX5VKrdGajG/0IrlkT3xwVNJVJSs4MpOHcw4 1o3BcV8OuH63EI4/DaISzfONMgNKgWqv43cR4icParaJPBeZsXbFhtI/hZ9hVPY5QBmJ4kPQOQo9+ /yrPBQTtijA7/1rRlDK6Douvko5c/dkV2Ky2jYh5sVdhTSuHkgq0S24m5GoQcx34KzJOkOg50L8SH +o+lKJ45URFR4xZn3YUppCurr5mmiHlhfZGl6DFTS8eI4elQrX2C4k5DVWe8JUmKi0dPAYw1xpnER vA/RBWKA==; Received: from willy by casper.infradead.org with local (Exim 4.94.2 #2 (Red Hat Linux)) id 1pHdMg-009MFh-CK; Tue, 17 Jan 2023 04:14:26 +0000 Date: Tue, 17 Jan 2023 04:14:26 +0000 From: Matthew Wilcox To: Hyeonggon Yoo <42.hyeyoo@gmail.com> Cc: Suren Baghdasaryan , 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 Subject: Re: [PATCH 41/41] mm: replace rw_semaphore with atomic_t in vma_lock Message-ID: References: <20230109205336.3665937-1-surenb@google.com> <20230109205336.3665937-42-surenb@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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.