linux-kernel - Re: [PATCH 2/5] fs: charge pipe buffers to memcg

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Message-Id: <20150929155711.3b139dab622848a14af64ca4@linux-foundation.org>
Date:	Tue, 29 Sep 2015 15:57:11 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Vladimir Davydov <vdavydov@...allels.com>
Cc:	Johannes Weiner <hannes@...xchg.org>,
	Michal Hocko <mhocko@...nel.org>, Tejun Heo <tj@...nel.org>,
	<linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/5] fs: charge pipe buffers to memcg

On Sat, 26 Sep 2015 13:45:54 +0300 Vladimir Davydov <vdavydov@...allels.com> wrote:

> Pipe buffers can be generated unrestrictedly by an unprivileged
> userspace process, so they shouldn't go unaccounted.
> 
> ...
>
> --- a/fs/pipe.c
> +++ b/fs/pipe.c
> @@ -400,7 +400,7 @@ pipe_write(struct kiocb *iocb, struct iov_iter *from)
>  			int copied;
>  
>  			if (!page) {
> -				page = alloc_page(GFP_HIGHUSER);
> +				page = alloc_kmem_pages(GFP_HIGHUSER, 0);
>  				if (unlikely(!page)) {
>  					ret = ret ? : -ENOMEM;
>  					break;

This seems broken.  We have a page buffer page which has a weird
->mapcount.  Now it gets stolen (generic_pipe_buf_steal()) and spliced
into pagecache.  Then the page gets mmapped and MM starts playing with
its ->_mapcount?


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