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=-1.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS 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 8357FC282CE for ; Wed, 13 Feb 2019 20:59:14 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 5C12F207E0 for ; Wed, 13 Feb 2019 20:59:14 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2394682AbfBMU7J (ORCPT ); Wed, 13 Feb 2019 15:59:09 -0500 Received: from mail.linuxfoundation.org ([140.211.169.12]:33480 "EHLO mail.linuxfoundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1731303AbfBMU7I (ORCPT ); Wed, 13 Feb 2019 15:59:08 -0500 Received: from akpm3.svl.corp.google.com (unknown [104.133.8.65]) by mail.linuxfoundation.org (Postfix) with ESMTPSA id 17357121F; Wed, 13 Feb 2019 20:59:07 +0000 (UTC) Date: Wed, 13 Feb 2019 12:59:06 -0800 From: Andrew Morton To: Jann Horn Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, Michal Hocko , Vlastimil Babka , Pavel Tatashin , Oscar Salvador , Mel Gorman , Aaron Lu , netdev@vger.kernel.org, Alexander Duyck Subject: Re: [PATCH] mm: page_alloc: fix ref bias in page_frag_alloc() for 1-byte allocs Message-Id: <20190213125906.eae96c18fe585e060aaf0ef7@linux-foundation.org> In-Reply-To: <20190213204157.12570-1-jannh@google.com> References: <20190213204157.12570-1-jannh@google.com> X-Mailer: Sylpheed 3.6.0 (GTK+ 2.24.31; x86_64-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: netdev-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: netdev@vger.kernel.org On Wed, 13 Feb 2019 21:41:57 +0100 Jann Horn wrote: > The basic idea behind ->pagecnt_bias is: If we pre-allocate the maximum > number of references that we might need to create in the fastpath later, > the bump-allocation fastpath only has to modify the non-atomic bias value > that tracks the number of extra references we hold instead of the atomic > refcount. The maximum number of allocations we can serve (under the > assumption that no allocation is made with size 0) is nc->size, so that's > the bias used. > > However, even when all memory in the allocation has been given away, a > reference to the page is still held; and in the `offset < 0` slowpath, the > page may be reused if everyone else has dropped their references. > This means that the necessary number of references is actually > `nc->size+1`. > > Luckily, from a quick grep, it looks like the only path that can call > page_frag_alloc(fragsz=1) is TAP with the IFF_NAPI_FRAGS flag, which > requires CAP_NET_ADMIN in the init namespace and is only intended to be > used for kernel testing and fuzzing. For the net-naive, what is TAP? It doesn't appear to mean drivers/net/tap.c. > To test for this issue, put a `WARN_ON(page_ref_count(page) == 0)` in the > `offset < 0` path, below the virt_to_page() call, and then repeatedly call > writev() on a TAP device with IFF_TAP|IFF_NO_PI|IFF_NAPI_FRAGS|IFF_NAPI, > with a vector consisting of 15 elements containing 1 byte each. > > ... > > --- a/mm/page_alloc.c > +++ b/mm/page_alloc.c > @@ -4675,11 +4675,11 @@ void *page_frag_alloc(struct page_frag_cache *nc, > /* Even if we own the page, we do not use atomic_set(). > * This would break get_page_unless_zero() users. > */ > - page_ref_add(page, size - 1); > + page_ref_add(page, size); > > /* reset page count bias and offset to start of new frag */ > nc->pfmemalloc = page_is_pfmemalloc(page); > - nc->pagecnt_bias = size; > + nc->pagecnt_bias = size + 1; > nc->offset = size; > } > > @@ -4695,10 +4695,10 @@ void *page_frag_alloc(struct page_frag_cache *nc, > size = nc->size; > #endif > /* OK, page count is 0, we can safely set it */ > - set_page_count(page, size); > + set_page_count(page, size + 1); > > /* reset page count bias and offset to start of new frag */ > - nc->pagecnt_bias = size; > + nc->pagecnt_bias = size + 1; > offset = size - fragsz; > } This is probably more a davem patch than a -mm one.