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

[Matthew Wilcox <willy@xxxxxxxxxxxxx>]

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

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:

+++ b/mm/gup.c
@@ -2754,6 +2754,7 @@ static int internal_get_user_pages_fast(unsigned long start, int nr_pages,
                                       FOLL_FORCE | FOLL_PIN | FOLL_GET)))
                return -EINVAL;
 
+       might_lock_read(&current->mm->mmap_lock);
        start = untagged_addr(start) & PAGE_MASK;
        addr = start;
        len = (unsigned long) nr_pages << PAGE_SHIFT;