On Mon, May 25, 2020 at 11:11:42AM -0400, Peter Xu wrote: > On Mon, May 25, 2020 at 11:46:51AM -0300, Jason Gunthorpe wrote: > > On Mon, May 25, 2020 at 10:28:06AM -0400, Peter Xu wrote: > > > On Mon, May 25, 2020 at 09:26:07AM -0300, Jason Gunthorpe wrote: > > > > On Sat, May 23, 2020 at 07:52:57PM -0400, Peter Xu wrote: > > > > > > > > > For what I understand now, IMHO we should still need all those handlings of > > > > > FAULT_FLAG_RETRY_NOWAIT like in the initial version. E.g., IIUC KVM gup will > > > > > try with FOLL_NOWAIT when async is allowed, before the complete slow path. I'm > > > > > not sure what would be the side effect of that if fault() blocked it. E.g., > > > > > the caller could be in an atomic context. > > > > > > > > AFAICT FAULT_FLAG_RETRY_NOWAIT only impacts what happens when > > > > VM_FAULT_RETRY is returned, which this doesn't do? > > > > > > Yes, that's why I think we should still properly return VM_FAULT_RETRY if > > > needed.. because IMHO it is still possible that the caller calls with > > > FAULT_FLAG_RETRY_NOWAIT. > > > > > > My understanding is that FAULT_FLAG_RETRY_NOWAIT majorly means: > > > > > > - We cannot release the mmap_sem, and, > > > - We cannot sleep > > > > Sleeping looks fine, look at any FS implementation of fault, say, > > xfs. The first thing it does is xfs_ilock() which does down_write(). > > Yeah. My wild guess is that maybe fs code will always be without > FAULT_FLAG_RETRY_NOWAIT so it's safe to sleep unconditionally (e.g., I think > the general #PF should be fine to sleep in fault(); gup should be special, but > I didn't observe any gup code called upon file systems)? get_user_pages is called on filesystem backed pages. I have no idea what FAULT_FLAG_RETRY_NOWAIT is supposed to do. Maybe John was able to guess when he reworked that stuff? Jason
