netdev - Re: [PATCH RESEND 7/8] pipe: account to kmemcg

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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 ?