Re: Question about PRIVATE_FUTEX

[Eric Dumazet <dada1@cosmosbay.com>]

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.