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Message-ID: <1464120273.5939.53.camel@edumazet-glaptop3.roam.corp.google.com> Date: Tue, 24 May 2016 13:04:33 -0700 From: Eric Dumazet <eric.dumazet@...il.com> To: Vladimir Davydov <vdavydov@...tuozzo.com> Cc: Andrew Morton <akpm@...ux-foundation.org>, Alexander Viro <viro@...iv.linux.org.uk>, Johannes Weiner <hannes@...xchg.org>, Michal Hocko <mhocko@...nel.org>, linux-mm@...ck.org, linux-fsdevel@...r.kernel.org, netdev@...r.kernel.org, x86@...nel.org, linux-kernel@...r.kernel.org Subject: Re: [PATCH RESEND 7/8] pipe: account to kmemcg 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 ?
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