From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-oi0-f70.google.com (mail-oi0-f70.google.com [209.85.218.70]) by kanga.kvack.org (Postfix) with ESMTP id AB42B6B0005 for ; Wed, 25 May 2016 06:30:24 -0400 (EDT) Received: by mail-oi0-f70.google.com with SMTP id r64so68012176oie.1 for ; Wed, 25 May 2016 03:30:24 -0700 (PDT) Received: from EUR01-DB5-obe.outbound.protection.outlook.com (mail-db5eur01on0115.outbound.protection.outlook.com. [104.47.2.115]) by mx.google.com with ESMTPS id b6si4708362otc.116.2016.05.25.03.30.23 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-SHA bits=128/128); Wed, 25 May 2016 03:30:23 -0700 (PDT) Date: Wed, 25 May 2016 13:30:11 +0300 From: Vladimir Davydov Subject: Re: [PATCH RESEND 7/8] pipe: account to kmemcg Message-ID: <20160525103011.GF11150@esperanza> References: <2c2545563b6201f118946f96dd8cfc90e564aff6.1464079538.git.vdavydov@virtuozzo.com> <1464094742.5939.46.camel@edumazet-glaptop3.roam.corp.google.com> <20160524161336.GA11150@esperanza> <1464120273.5939.53.camel@edumazet-glaptop3.roam.corp.google.com> MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Disposition: inline In-Reply-To: <1464120273.5939.53.camel@edumazet-glaptop3.roam.corp.google.com> Sender: owner-linux-mm@kvack.org List-ID: To: Eric Dumazet Cc: Andrew Morton , Alexander Viro , Johannes Weiner , Michal Hocko , linux-mm@kvack.org, linux-fsdevel@vger.kernel.org, netdev@vger.kernel.org, x86@kernel.org, linux-kernel@vger.kernel.org On Tue, May 24, 2016 at 01:04:33PM -0700, Eric Dumazet wrote: > On Tue, 2016-05-24 at 19:13 +0300, Vladimir Davydov wrote: > > On Tue, May 24, 2016 at 05:59:02AM -0700, Eric Dumazet wrote: > > ... > > > > +static int anon_pipe_buf_steal(struct pipe_inode_info *pipe, > > > > + struct pipe_buffer *buf) > > > > +{ > > > > + struct page *page = buf->page; > > > > + > > > > + if (page_count(page) == 1) { > > > > > > This looks racy : some cpu could have temporarily elevated page count. > > > > All pipe operations (pipe_buf_operations->get, ->release, ->steal) are > > supposed to be called under pipe_lock. So, if we see a pipe_buffer->page > > with refcount of 1 in ->steal, that means that we are the only its user > > and it can't be spliced to another pipe. > > > > In fact, I just copied the code from generic_pipe_buf_steal, adding > > kmemcg related checks along the way, so it should be fine. > > So you guarantee that no other cpu might have done > get_page_unless_zero() right before this test ? Each pipe_buffer holds a reference to its page. If we find page's refcount to be 1 here, then it can be referenced only by our pipe_buffer. And the refcount cannot be increased by a parallel thread, because we hold pipe_lock, which rules out splice, and otherwise it's impossible to reach the page as it is not on lru. That said, I think I guarantee that this should be safe. Thanks, Vladimir -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org