linux-kernel - Re: Question about PRIVATE

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Message-ID: <49CC5C7A.9070505@cosmosbay.com>
Date:	Fri, 27 Mar 2009 05:56:26 +0100
From:	Eric Dumazet <dada1@...mosbay.com>
To:	Minchan Kim <minchan.kim@...il.com>
CC:	Peter Zijlstra <peterz@...radead.org>,
	lkml <linux-kernel@...r.kernel.org>
Subject: Re: Question about PRIVATE_FUTEX

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.


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