Re: Question about PRIVATE_FUTEX

[Peter Zijlstra <peterz@infradead.org>]

On Fri, 2009-03-27 at 11:12 +0900, Minchan Kim wrote:
> 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.

But also drops that page ref at the end of get_futex_key(). The whole
and only purpose of using get_user_pages_fast() is to get at the mapping
data without having to obtain the mmap_sem.

> 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 ?

Nobody, all uses of get_futex_value_locked() have to deal with it
returning -EFAULT.

Most of this is legacy btw, from when futex ops were done under the
mmap_sem. Back then we couldn't fault because that would cause mmap_sem
recursion. Howver, now that we don't hold mmap_sem anymore we could use
a faulting user access like get_user().

Darren has been working on patches to clean that up, some of those are
already merged in the -tip tree.

HTH