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 X-Spam-Level: X-Spam-Status: No, score=-5.4 required=3.0 tests=DKIM_SIGNED,DKIM_VALID, DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_2 autolearn=unavailable autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 55228C352A5 for ; Mon, 10 Feb 2020 19:58:22 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id E8D6220838 for ; Mon, 10 Feb 2020 19:58:21 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=lca.pw header.i=@lca.pw header.b="JNQbuvAH" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org E8D6220838 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=lca.pw Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id 4EBEE6B0160; Mon, 10 Feb 2020 14:58:21 -0500 (EST) Received: by kanga.kvack.org (Postfix, from userid 40) id 49D8E6B0161; Mon, 10 Feb 2020 14:58:21 -0500 (EST) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 38BDA6B0162; Mon, 10 Feb 2020 14:58:21 -0500 (EST) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0162.hostedemail.com [216.40.44.162]) by kanga.kvack.org (Postfix) with ESMTP id 206306B0160 for ; Mon, 10 Feb 2020 14:58:21 -0500 (EST) Received: from smtpin21.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay05.hostedemail.com (Postfix) with ESMTP id B3948181AC9CC for ; Mon, 10 Feb 2020 19:58:20 +0000 (UTC) X-FDA: 76475279160.21.brass97_65ead2f9c0859 X-HE-Tag: brass97_65ead2f9c0859 X-Filterd-Recvd-Size: 6650 Received: from mail-qk1-f194.google.com (mail-qk1-f194.google.com [209.85.222.194]) by imf31.hostedemail.com (Postfix) with ESMTP for ; Mon, 10 Feb 2020 19:58:20 +0000 (UTC) Received: by mail-qk1-f194.google.com with SMTP id w25so7825744qki.3 for ; Mon, 10 Feb 2020 11:58:20 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=lca.pw; s=google; h=message-id:subject:from:to:cc:date:in-reply-to:references :mime-version:content-transfer-encoding; bh=oYGFsd3OXEi9GKcbRC9zUa47GEfhq/gdmQM5+oaRjMk=; b=JNQbuvAHB+LvQM63jiL2mV4AuV62yR92wHEYGBYWqjlVlwwWaa2VuIuFNfIVV9AxxM LCc7p8dAknwSBHPaUpGeJTc4Cl+Bc/9Cow3P8M6UsfUVFJ8/XDqtcMdLKvJz91MNPomm oXCfpEjxAi/Z46OUHFWKQ7RfXn6MFouEskXfaU2fNg5A43G++snsPNRB5zfkd+dvJBQ/ DPntkfr8JpdFrIRQMPt9ahOZvUDPmy3Fz1dO7JZNAp6lRsEUXVAq+a1H1o+x4C3khhXA MjgyV+re2JYttcQsdYvdFjRgC4KsKNYm+8gQ0sAo3lYBeJMJuRC3ylrSDzw7CtlBv2s3 LBPA== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:message-id:subject:from:to:cc:date:in-reply-to :references:mime-version:content-transfer-encoding; bh=oYGFsd3OXEi9GKcbRC9zUa47GEfhq/gdmQM5+oaRjMk=; b=iVORr5kJnLOd6m3LocrhrC/VllYakV+n3t0scLEFj9YgNzA3jmdJeF5CmTEbYYOpdl Hv9PbVXywBHX1ejaiB+jgUAVkF7iatPXetlB2QbrGA5WbY5JQE9ENz0DYA5n69rnyeQj WBk02zyFlcFrwz22HJyfy61aOu2vY2uDLIlbclqIim0CLv15w9KLcG4ZLi5TBtaZhblP 1+bdTB5ybokrNsNutzHm8UXG4APw8CXmvq4VX/vnj0tuYkypYF+8uVntPxNj4nCC2szU ISHHY/Ld3+R9Bti5EYNv19PmcCjduko4nFqdLpc6kr4LZSy4jw9poNa+ZYsz6j9PJ5M+ u/fA== X-Gm-Message-State: APjAAAXWF3UgyrjhWwiEiClLCpj1IwBqu2bNZtEO1UJgGeUMX7zXTOnL NP3rXm9mQQZ1vpw58TxikFhXeMuj67HbcQ== X-Google-Smtp-Source: APXvYqzaTnBGc+oGvMokoQzM/TDVbYoBgxsUSjJ67hXHMSrH+U1maZqQOSWoGBk9HcC5iQ4M24fHMA== X-Received: by 2002:a37:8e03:: with SMTP id q3mr2918928qkd.395.1581364699626; Mon, 10 Feb 2020 11:58:19 -0800 (PST) Received: from dhcp-41-57.bos.redhat.com (nat-pool-bos-t.redhat.com. [66.187.233.206]) by smtp.gmail.com with ESMTPSA id w16sm627632qkj.135.2020.02.10.11.58.17 (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Mon, 10 Feb 2020 11:58:18 -0800 (PST) Message-ID: <1581364697.7365.45.camel@lca.pw> Subject: Re: [PATCH -next] mm/filemap: fix a data race in filemap_fault() From: Qian Cai To: "Kirill A. Shutemov" , Matthew Wilcox Cc: akpm@linux-foundation.org, elver@google.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org Date: Mon, 10 Feb 2020 14:58:17 -0500 In-Reply-To: <20200210180546.vt7yhdjav5oinij7@box> References: <1581354029-20154-1-git-send-email-cai@lca.pw> <20200210172511.GL8731@bombadil.infradead.org> <20200210180546.vt7yhdjav5oinij7@box> Content-Type: text/plain; charset="UTF-8" X-Mailer: Evolution 3.22.6 (3.22.6-10.el7) Mime-Version: 1.0 Content-Transfer-Encoding: 7bit X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Mon, 2020-02-10 at 21:05 +0300, Kirill A. Shutemov wrote: > On Mon, Feb 10, 2020 at 09:25:11AM -0800, Matthew Wilcox wrote: > > On Mon, Feb 10, 2020 at 12:00:29PM -0500, Qian Cai wrote: > > > @@ -2622,7 +2622,7 @@ void filemap_map_pages(struct vm_fault *vmf, > > > if (page->index >= max_idx) > > > goto unlock; > > > > > > - if (file->f_ra.mmap_miss > 0) > > > + if (data_race(file->f_ra.mmap_miss > 0)) > > > file->f_ra.mmap_miss--; > > > > How is this safe? Two threads can each see 1, and then both decrement the > > in-memory copy, causing it to end up at -1. > > Right, it is bogus. > > Below is my completely untested attempt on fix this. It still allows > races, but they will only lead to missed accounting, but not underflow. Looks good to me. Do you plan to send out an official patch? > > > diff --git a/mm/filemap.c b/mm/filemap.c > index 1784478270e1..1919d37c646a 100644 > --- a/mm/filemap.c > +++ b/mm/filemap.c > @@ -2365,6 +2365,7 @@ static struct file *do_sync_mmap_readahead(struct vm_fault *vmf) > struct address_space *mapping = file->f_mapping; > struct file *fpin = NULL; > pgoff_t offset = vmf->pgoff; > + unsigned mmap_miss; > > /* If we don't want any read-ahead, don't bother */ > if (vmf->vma->vm_flags & VM_RAND_READ) > @@ -2380,14 +2381,15 @@ static struct file *do_sync_mmap_readahead(struct vm_fault *vmf) > } > > /* Avoid banging the cache line if not needed */ > - if (ra->mmap_miss < MMAP_LOTSAMISS * 10) > - ra->mmap_miss++; > + mmap_miss = READ_ONCE(ra->mmap_miss); > + if (mmap_miss < MMAP_LOTSAMISS * 10) > + WRITE_ONCE(ra->mmap_miss, ++mmap_miss); > > /* > * Do we miss much more than hit in this file? If so, > * stop bothering with read-ahead. It will only hurt. > */ > - if (ra->mmap_miss > MMAP_LOTSAMISS) > + if (mmap_miss > MMAP_LOTSAMISS) > return fpin; > > /* > @@ -2413,13 +2415,15 @@ static struct file *do_async_mmap_readahead(struct vm_fault *vmf, > struct file_ra_state *ra = &file->f_ra; > struct address_space *mapping = file->f_mapping; > struct file *fpin = NULL; > + unsigned int mmap_miss; > pgoff_t offset = vmf->pgoff; > > /* If we don't want any read-ahead, don't bother */ > if (vmf->vma->vm_flags & VM_RAND_READ) > return fpin; > - if (ra->mmap_miss > 0) > - ra->mmap_miss--; > + mmap_miss = READ_ONCE(ra->mmap_miss); > + if (mmap_miss) > + WRITE_ONCE(ra->mmap_miss, --mmap_miss); > if (PageReadahead(page)) { > fpin = maybe_unlock_mmap_for_io(vmf, fpin); > page_cache_async_readahead(mapping, ra, file, > @@ -2586,7 +2590,9 @@ void filemap_map_pages(struct vm_fault *vmf, > unsigned long max_idx; > XA_STATE(xas, &mapping->i_pages, start_pgoff); > struct page *page; > + unsigned long mmap_miss; > > + mmap_miss = READ_ONCE(file->f_ra.mmap_miss); > rcu_read_lock(); > xas_for_each(&xas, page, end_pgoff) { > if (xas_retry(&xas, page)) > @@ -2622,8 +2628,8 @@ void filemap_map_pages(struct vm_fault *vmf, > if (page->index >= max_idx) > goto unlock; > > - if (file->f_ra.mmap_miss > 0) > - file->f_ra.mmap_miss--; > + if (mmap_miss > 0) > + mmap_miss--; > > vmf->address += (xas.xa_index - last_pgoff) << PAGE_SHIFT; > if (vmf->pte) > @@ -2643,6 +2649,7 @@ void filemap_map_pages(struct vm_fault *vmf, > break; > } > rcu_read_unlock(); > + WRITE_ONCE(file->f_ra.mmap_miss, mmap_miss); > } > EXPORT_SYMBOL(filemap_map_pages); >