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@xxxxxxxxxx
[2] https://lore.kernel.org/r/20200518054315.2407093-1-jhubbard@xxxxxxxxxx
thanks,
--
John Hubbard
NVIDIA
