On Wed 26-10-16 10:39:13, Lorenzo Stoakes wrote: > 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. Ohh, right. I should have checked more closely. > 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. I do not see any reason why get_user_pages_remote should implicitely disallow VM_FAULT_RETRY. Releasing the mmap_sem on a remote task when we have to wait for IO is a good thing in general. So I would rather see a way to do allow that. Doing that implicitly sounds too dangerous and maybe we even have users which wouldn't cope with the mmap sem being dropped (get_arg_page sounds like a potential example) so I would rather add locked * parameter to get_user_pages_remote. -- Michal Hocko SUSE Labs -- 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
