linux-kernel - Re: [PATCH V2] fs: don't write pages when receiving a pending SIGKILL in __get_user

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Message-ID: <alpine.DEB.2.02.1401202151500.21729@chino.kir.corp.google.com>
Date:	Mon, 20 Jan 2014 21:53:42 -0800 (PST)
From:	David Rientjes <rientjes@...gle.com>
To:	Xishi Qiu <qiuxishi@...wei.com>
cc:	robin.yb@...wei.com, viro@...iv.linux.org.uk,
	linux-fsdevel@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH V2] fs: don't write pages when receiving a pending SIGKILL
 in __get_user_pages()

On Sat, 18 Jan 2014, Xishi Qiu wrote:

> In the process IO direction, dio_refill_pages will call get_user_pages_fast 
> to map the page from user space. If ret is less than 0 and IO is write, the 
> function will create a zero page to fill data. This may work for some file 
> system, but in some device operate we prefer whole write or fail, not half 
> data half zero, e.g. fs metadata, like inode, identy.

So you're attemping to define a behavior for all users of direct IO for a 
problem that is filesystem or backing device dependent?  Perhaps if you 
elaborated on the problem that you're seeing then we could address it.

> This happens often when kill a process which is doing direct IO. Consider 
> the following cases, the process A is doing IO process, may enter __get_user_pages 
> function, if other processes send process A SIG_KILL, A will enter the 
> following branches 
> 		/*
> 		 * If we have a pending SIGKILL, don't keep faulting
> 		 * pages and potentially allocating memory.
> 		 */
> 		if (unlikely(fatal_signal_pending(current)))
> 			return i ? i : -ERESTARTSYS;
> Return current pages. direct IO will write the pages, the subsequent pages 
> which can’t get will use zero page instead. 
> 
> Signed-off-by: Xishi Qiu <qiuxishi@...wei.com>
> Signed-off-by: Bin Yang <robin.yb@...wei.com>
> ---
>  fs/direct-io.c |    3 +++
>  1 files changed, 3 insertions(+), 0 deletions(-)
> 
> diff --git a/fs/direct-io.c b/fs/direct-io.c
> index 0e04142..b74d565 100644
> --- a/fs/direct-io.c
> +++ b/fs/direct-io.c
> @@ -174,6 +174,9 @@ static inline int dio_refill_pages(struct dio *dio, struct dio_submit *sdio)
>  		&dio->pages[0]);		/* Put results here */
>  
>  	if (ret < 0 && sdio->blocks_available && (dio->rw & WRITE)) {
> +		/* If task is killed, do not write anymore */
> +		if (ret == -ERESTARTSYS)
> +			goto out;
>  		struct page *page = ZERO_PAGE(0);
>  		/*
>  		 * A memory fault, but the filesystem has some outstanding

We don't mix declarations and text, please try to compile your patches 
before proposing them.