Re: [PATCH v5.5 10/10] mmap locking API: rename mmap_sem to mmap_lock

[Michel Lespinasse <walken@google.com>]

On Tue, May 19, 2020 at 11:15 AM John Hubbard <jhubbard@nvidia.com> wrote:
> On 2020-05-19 08:32, Matthew Wilcox wrote:
> > On Tue, May 19, 2020 at 03:20:40PM +0200, Laurent Dufour wrote:
> >> Le 19/05/2020 à 15:10, Michel Lespinasse a écrit :
> >>> On Mon, May 18, 2020 at 03:45:22PM +0200, Laurent Dufour wrote:
> >>>> Le 24/04/2020 à 03:39, Michel Lespinasse a écrit :
> >>>>> Rename the mmap_sem field to mmap_lock. Any new uses of this lock
> >>>>> should now go through the new mmap locking api. The mmap_lock is
> >>>>> still implemented as a rwsem, though this could change in the future.
> >>>>>
> >>>>> diff --git a/drivers/gpu/drm/etnaviv/etnaviv_gem.c b/drivers/gpu/drm/etnaviv/etnaviv_gem.c
> >>>>> index dc9ef302f517..701f3995f621 100644
> >>>>> --- a/drivers/gpu/drm/etnaviv/etnaviv_gem.c
> >>>>> +++ b/drivers/gpu/drm/etnaviv/etnaviv_gem.c
> >>>>> @@ -661,7 +661,7 @@ static int etnaviv_gem_userptr_get_pages(struct etnaviv_gem_object *etnaviv_obj)
> >>>>>           struct etnaviv_gem_userptr *userptr = &etnaviv_obj->userptr;
> >>>>>           int ret, pinned = 0, npages = etnaviv_obj->base.size >> PAGE_SHIFT;
> >>>>> - might_lock_read(&current->mm->mmap_sem);
> >>>>> + might_lock_read(&current->mm->mmap_lock);
> >>>>
> >>>> Why not a mm_might_lock_read() new API to hide the mmap_lock, and add it to
> >>>> the previous patch?
> >>>
> >>> I'm not sure why this is needed - we may rework the lock to be
> >>> something else than rwsem, but might_lock_read should still apply to
> >>> it and make sense ? I'm not sure what the extra API would bring...
> >>
> >> I guess at one time the API would become might_lock_read_a_range(), isn't it?

I don't think this should be necessary - from lockdep perspective,
there should not be a difference between locking a range vs the entire
address space.

> >> Furthermore this would hiding the lock's name which the goal of this series.

Actually to me the name is very secondary - the goal of the series is
to add an api abstracting the mmap locking. The lock name is secondary
to that, it only gets renamed because mmap_sem was too specific (if we
are to change the mmap locking) and to ensure we convert all direct
uses to use the api instead.

> > I think this assertion should be deleted from this driver.  It's there
> > in case get_user_pages_fast() takes the mmap sem.  It would make sense to
> > have this assertion in get_user_pages_fast() in case we take the fast path
> > which doesn't acquire the mmap_sem.  Something like this:

I like this idea a lot - having might_lock assertions in
get_user_pages_fast makes a log more sense than doing the same at the
call sites.

> There are a couple of recent developments in this code to keep in mind. I don't
> *think* either one is a problem here, but just in case:
>
> a) The latest version of the above routine [1] is on its way to mmotm as of
> yesterday, and that version more firmly divides the fast and slow parts,
> via a new FOLL_FAST_ONLY flag. The fall-back to slow/regular gup only occurs
> if the caller does not set FOLL_FAST_ONLY. (Note that it's a gup.c internal
> flag, btw.)
>
> That gives you additional options inside internal_get_user_pages_fast(), such
> as, approximately:
>
> if (!(gup_flags & FOLL_FAST_ONLY))
>         might_lock_read(&current->mm->mmap_lock);
>
> ...not that that is necessarily a great idea, seeing as how it merely changes
> "might lock" into "maybe might lock".  :)

I think that is completely fine, makes sure everyone not using
FOLL_FAST_ONLY realizes that the call could block.

Can I ask you to add that assertion in your patchset ? Based on
Matthew's feedback, I would do it in my patchset, but it doesn't seem
worth doing if we know this will conflict with your changes.

-- 
Michel "Walken" Lespinasse
A program is never fully debugged until the last user dies.