On Wed, Aug 14, 2019 at 02:38:30PM -0300, Jason Gunthorpe wrote: > On Wed, Aug 14, 2019 at 11:00:34PM +0530, Bharath Vedartham wrote: > > On Tue, Aug 13, 2019 at 01:19:38PM -0500, Dimitri Sivanich wrote: > > > On Tue, Aug 13, 2019 at 10:53:01PM +0530, Bharath Vedartham wrote: > > > > On Tue, Aug 13, 2019 at 09:50:29AM -0500, Dimitri Sivanich wrote: > > > > > Bharath, > > > > > > > > > > I do not believe that __get_user_pages_fast will work for the atomic case, as > > > > > there is no guarantee that the 'current->mm' will be the correct one for the > > > > > process in question, as the process might have moved away from the cpu that is > > > > > handling interrupts for it's context. > > > > So what your saying is, there may be cases where current->mm != gts->ts_mm > > > > right? __get_user_pages_fast and get_user_pages do assume current->mm. > > > > > > Correct, in the case of atomic context. > > > > > > > > > > > These changes were inspired a bit from kvm. In kvm/kvm_main.c, > > > > hva_to_pfn_fast uses __get_user_pages_fast. THe comment above the > > > > function states it runs in atomic context. > > > > > > > > Just curious, get_user_pages also uses current->mm. Do you think that is > > > > also an issue? > > > > > > Not in non-atomic context. Notice that it is currently done that way. > > > > > > > > > > > Do you feel using get_user_pages_remote would be a better idea? We can > > > > specify the mm_struct in get_user_pages_remote? > > > > > > From that standpoint maybe, but is it safe in interrupt context? > > Hmm.. The gup maintainers seemed fine with the code.. > > > > Now this is only an issue if gru_vtop can be executed in an interrupt > > context. > > > > get_user_pages_remote is not valid in an interrupt context(if CONFIG_MMU > > is set). If we follow the function, in __get_user_pages, cond_resched() > > is called which definitly confirms that we can't run this function in an > > interrupt context. > > > > I think we might need some advice from the gup maintainers here. > > Note that the comment on the function __get_user_pages_fast states that > > __get_user_pages_fast is IRQ-safe. > > vhost is doing some approach where they switch current to the target > then call __get_user_pages_fast in an IRQ context, that might be a > reasonable pattern > > If this is a regular occurance we should probably add a > get_atomic_user_pages_remote() to make the pattern clear. > > Jason That makes sense. get_atomic_user_pages_remote() should not be hard to write. AFAIKS __get_user_pages_fast is special_cased for current, we could probably just add a new parameter of the mm_struct to the page table walking code in gup.c But till then I think we can approach this by the way vhost approaches this problem by switching current to the target. Thank you Bharath
