Re: [PATCH] mm: remove unnecessary __get_user_pages_unlocked() calls

[Michal Hocko]

On Wed 26-10-16 10:39:13, Lorenzo Stoakes wrote:
> On Wed, Oct 26, 2016 at 11:15:43AM +0200, Michal Hocko wrote:
> > On Wed 26-10-16 00:46:31, Lorenzo Stoakes wrote:
> > > The holdout for unexporting __get_user_pages_unlocked() is its invocation in
> > > mm/process_vm_access.c: process_vm_rw_single_vec(), as this definitely _does_
> > > seem to invoke VM_FAULT_RETRY behaviour which get_user_pages_remote() will not
> > > trigger if we were to replace it with the latter.
> >
> > I am not sure I understand. Prior to 1e9877902dc7e this used
> > get_user_pages_unlocked. What prevents us from reintroducing it with
> > FOLL_REMOVE which was meant to be added by the above commit?
> >
> > Or am I missing your point?
> 
> The issue isn't the flags being passed, rather that in this case:
> 
> a. Replacing __get_user_pages_unlocked() with get_user_pages_unlocked() won't
>    work as the latter assumes task = current and mm = current->mm but
>    process_vm_rw_single_vec() needs to pass different task, mm.

Ohh, right. I should have checked more closely.

> b. Moving to get_user_pages_remote() _will_ allow us to pass different task, mm
>    but won't however match existing behaviour precisely, since
>    __get_user_pages_unlocked() acquires mmap_sem then passes a pointer to a
>    local 'locked' variable to __get_user_pages_locked() which allows
>    VM_FAULT_RETRY to trigger.

I do not see any reason why get_user_pages_remote should implicitely
disallow VM_FAULT_RETRY. Releasing the mmap_sem on a remote task when we
have to wait for IO is a good thing in general. So I would rather see a
way to do allow that. Doing that implicitly sounds too dangerous and
maybe we even have users which wouldn't cope with the mmap sem being
dropped (get_arg_page sounds like a potential example) so I would rather
add locked * parameter to get_user_pages_remote.

-- 
Michal Hocko
SUSE Labs