linux-kernel - Re: Question about PRIVATE

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Date:	Fri, 27 Mar 2009 14:20:12 +0900
From:	Minchan Kim <minchan.kim@...il.com>
To:	Eric Dumazet <dada1@...mosbay.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	lkml <linux-kernel@...r.kernel.org>
Subject: Re: Question about PRIVATE_FUTEX

Thanks for kind explanation.

On Fri, Mar 27, 2009 at 1:56 PM, Eric Dumazet <dada1@...mosbay.com> wrote:
> Minchan Kim a écrit :
>> Hi, Peter and Eric.
>>
>> I am not expert about futex.
>> I am sorry if this is dumb question.
>>
>> If we use private futex, get_futex_key don't call get_user_pages_fast
>> which pins page at page table.
>> Then, get_futex_value_locked calls __cpy_from_user_inatomic with
>> pagefault_disable.
>>
>> Who make sure the user page is mapped at app's page table ?
>>
>
> Nothing makes sure user page is mapped, as we dont have to (for private futexes
> at least, since the 'key' is a combination of the futex virtual address (not
> depending on the underlying physical page) and the task mm (sort of a static
> offset per task)
> If no page is mapped, a normal error should be returned to user, since
> access to futex location will trigger a fault.
>

I mean as follows.
It seems even shared futex case.

After calling get_user_pages_fast, get_futex_key calls unlock_page and
put_page, too.  Then futex_wait  calls get_futex_value_locked.

Generally, current page->count is one and nolocked.
I think kernel reclaimer can reclaim the page.

Wouldn't kernel reclaim the page between get_fuex_key and
get_futex_value_locked ?
If kernel reclaimed the page, __copy_from_user_inatomic can happens
page fault although pagefault_disable is on.

How do we make sure this race condition ?
Do I miss something ?

-- 
Kinds regards,
Minchan Kim
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