Re: [PATCH] mm/fs: don't keep pages when receiving a pending SIGKILLin __get_user_pages()

From: Xishi Qiu
Date: Thu Jan 16 2014 - 08:00:16 EST

On 2014/1/16 7:15, David Rientjes wrote:

> On Wed, 15 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.
>> 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.
>> This patch will modify this judgment, if receive SIG_KILL, release pages and
>> return an error. Direct IO will find no blocks_available and return error
>> direct, rather than half IO data and half zero page.
>> Signed-off-by: Xishi Qiu <qiuxishi@xxxxxxxxxx>
>> Signed-off-by: Bin Yang <robin.yb@xxxxxxxxxx>
> It's scary to change the behavior of gup when some callers may want the
> exact opposite of what you're intending here, which is sane fallback by
> mapping the zero page. In fact, gup never does put_page() itself and
> __get_user_pages() always returns the number of pages pinned and may not
> equal what is passed.
> So, this definitely isn't the right solution for a special-case direct IO.
> Instead, it would be better to code this directly in the caller and
> compare the return value with nr_pages in dio_refill_pages() and then do
> the put_page() itself before falling back to ZERO_PAGE().

Hi Rientjes,
You are right, we should not change the behavior of gup.
I have a question, if we only get a part of the pages from get_user_pages_fast(),
shall we write them to the disk? or add a check before write?
I'm not familiar with fs.


Xishi Qiu

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