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

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Date:	Tue, 24 May 2016 19:13:36 +0300
From:	Vladimir Davydov <vdavydov@...tuozzo.com>
To:	Eric Dumazet <eric.dumazet@...il.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, 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.

Thanks,
Vladimir

> 
> > +		if (memcg_kmem_enabled()) {
> > +			memcg_kmem_uncharge(page, 0);
> > +			__ClearPageKmemcg(page);
> > +		}
> > +		__SetPageLocked(page);
> > +		return 0;
> > +	}
> > +	return 1;
> > +}