On Wed, Oct 26, 2016 at 11:15:43AM +0200, Michal Hocko wrote: > On Wed 26-10-16 00:46:31, Lorenzo Stoakes wrote: > > The holdout for unexporting __get_user_pages_unlocked() is its invocation in > > mm/process_vm_access.c: process_vm_rw_single_vec(), as this definitely _does_ > > seem to invoke VM_FAULT_RETRY behaviour which get_user_pages_remote() will not > > trigger if we were to replace it with the latter. > > I am not sure I understand. Prior to 1e9877902dc7e this used > get_user_pages_unlocked. What prevents us from reintroducing it with > FOLL_REMOVE which was meant to be added by the above commit? > > Or am I missing your point? The issue isn't the flags being passed, rather that in this case: a. Replacing __get_user_pages_unlocked() with get_user_pages_unlocked() won't work as the latter assumes task = current and mm = current->mm but process_vm_rw_single_vec() needs to pass different task, mm. b. Moving to get_user_pages_remote() _will_ allow us to pass different task, mm but won't however match existing behaviour precisely, since __get_user_pages_unlocked() acquires mmap_sem then passes a pointer to a local 'locked' variable to __get_user_pages_locked() which allows VM_FAULT_RETRY to trigger. The main issue I had here was not being sure whether we care about the VM_FAULT_RETRY functionality being used here or not, if we don't care then we can just move to get_user_pages_remote(), otherwise perhaps this should be left alone or maybe we need to consider adjusting the API to allow for remote access with VM_FAULT_RETRY functionality. -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
