On 11/11/2019 14:32, Chris Wilson wrote:
Quoting Chris Wilson (2019-11-11 14:27:16)
Quoting Tvrtko Ursulin (2019-11-11 14:19:31)
On 11/11/2019 13:32, Chris Wilson wrote:
Enable gup to retry and fault the pages outside of the mmap_sem lock in
our worker. As we are inside our worker, outside of any critical path,
we can allow the mmap_sem lock to be dropped in order to service a page
fault; this in turn allows the mm to populate the page using a slow
fault handler.
Testcase: igt/gem_userptr/userfault
There are no references or explanation on what does this fix?
gup simply fails if it is not allowed to drop the lock for some faults,
__get_user_pages_locked:
ret = __get_user_pages(tsk, mm, start, nr_pages, flags, pages,
vmas, locked);
if (!locked)
/* VM_FAULT_RETRY couldn't trigger, bypass */
return ret;
userfault being the first time I discovered this even existed. Since we
are only holding the mmap_sem for the gup (and not protecting anything
else) we can simply allow gup to drop the lock if it needs to.
Fixes: 5b56d49fc31d ("mm: add locked parameter to get_user_pages_remote()")
s/Fixes/Reference/ I guess.
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>
Regards,
Tvrtko
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