Messages in this thread | | | Subject | Re: [PATCH v5.5 10/10] mmap locking API: rename mmap_sem to mmap_lock | From | John Hubbard <> | Date | Tue, 19 May 2020 11:14:57 -0700 |
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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? >> 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(¤t->mm->mmap_lock); > start = untagged_addr(start) & PAGE_MASK; > addr = start; > len = (unsigned long) nr_pages << PAGE_SHIFT; > >
Hi Michel and Matthew and all,
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". :)
b) I've posted a small patch to that same etnaviv_gem.c file [2], over the weekend, to convert from get_user_pages()/put_page(), to pin_user_pages()/unpin_user_pages(). It hasn't been merged yet, and it shouldn't conflict either, but just one more reason to hope that the etnaviv list can do some run time testing on the whole lot.
[1] https://lore.kernel.org/r/20200519002124.2025955-3-jhubbard@nvidia.com
[2] https://lore.kernel.org/r/20200518054315.2407093-1-jhubbard@nvidia.com
thanks, -- John Hubbard NVIDIA
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