On Tue, May 19, 2020 at 11:15 AM John Hubbard <jhubbard@xxxxxxxxxx> 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(¤t->mm->mmap_sem); > >>>>> + might_lock_read(¤t->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(¤t->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.
