On Wed, Mar 20, 2019 at 12:33:20AM -0400, Jerome Glisse wrote: > On Tue, Mar 19, 2019 at 06:43:45PM -0700, John Hubbard wrote: > > On 3/19/19 5:08 PM, Jerome Glisse wrote: > > > On Wed, Mar 20, 2019 at 10:57:52AM +1100, Dave Chinner wrote: > > >> On Tue, Mar 19, 2019 at 06:06:55PM -0400, Jerome Glisse wrote: > > >>> On Wed, Mar 20, 2019 at 08:23:46AM +1100, Dave Chinner wrote: > > >>>> On Tue, Mar 19, 2019 at 10:14:16AM -0400, Jerome Glisse wrote: > > >>>>> On Tue, Mar 19, 2019 at 09:47:24AM -0400, Jerome Glisse wrote: > > >>>>>> On Tue, Mar 19, 2019 at 03:04:17PM +0300, Kirill A. Shutemov wrote: > > >>>>>>> On Fri, Mar 08, 2019 at 01:36:33PM -0800, john.hubbard@xxxxxxxxx wrote: > > >>>>>>>> From: John Hubbard <jhubbard@xxxxxxxxxx> > > >>>>>> [...] > > >>>>> Forgot to mention one thing, we had a discussion with Andrea and Jan > > >>>>> about set_page_dirty() and Andrea had the good idea of maybe doing > > >>>>> the set_page_dirty() at GUP time (when GUP with write) not when the > > >>>>> GUP user calls put_page(). We can do that by setting the dirty bit > > >>>>> in the pte for instance. They are few bonus of doing things that way: > > >>>>> - amortize the cost of calling set_page_dirty() (ie one call for > > >>>>> GUP and page_mkclean() > > >>>>> - it is always safe to do so at GUP time (ie the pte has write > > >>>>> permission and thus the page is in correct state) > > >>>>> - safe from truncate race > > >>>>> - no need to ever lock the page > > >>>> > > >>>> I seem to have missed this conversation, so please excuse me for > > >>> > > >>> The set_page_dirty() at GUP was in a private discussion (it started > > >>> on another topic and drifted away to set_page_dirty()). > > >>> > > >>>> asking a stupid question: if it's a file backed page, what prevents > > >>>> background writeback from cleaning the dirty page ~30s into a long > > >>>> term pin? i.e. I don't see anything in this proposal that prevents > > >>>> the page from being cleaned by writeback and putting us straight > > >>>> back into the situation where a long term RDMA is writing to a clean > > >>>> page.... > > >>> > > >>> So this patchset does not solve this issue. > > >> > > >> OK, so it just kicks the can further down the road. > > >> > > >>> [3..N] decide what to do for GUPed page, so far the plans seems > > >>> to be to keep the page always dirty and never allow page > > >>> write back to restore the page in a clean state. This does > > >>> disable thing like COW and other fs feature but at least > > >>> it seems to be the best thing we can do. > > >> > > >> So the plan for GUP vs writeback so far is "break fsync()"? :) > > >> > > >> We might need to work on that a bit more... > > > > > > Sorry forgot to say that we still do write back using a bounce page > > > so that at least we write something to disk that is just a snapshot > > > of the GUPed page everytime writeback kicks in (so either through > > > radix tree dirty page write back or fsync or any other sync events). > > > So many little details that i forgot the big chunk :) > > > > > > Cheers, > > > Jérôme > > > > > > > Dave, Jan, Jerome, > > > > Bounce pages for periodic data integrity still seem viable. But for the > > question of things like fsync or truncate, I think we were zeroing in > > on file leases as a nice building block. > > > > Can we revive the file lease discussion? By going all the way out to user > > space and requiring file leases to be coordinated at a high level in the > > software call chain, it seems like we could routinely avoid some of the > > worst conflicts that the kernel code has to resolve. > > > > For example: > > > > Process A > > ========= > > gets a lease on file_a that allows gup > > usage on a range within file_a > > > > sets up writable DMA: > > get_user_pages() on the file_a range > > start DMA (independent hardware ops) > > hw is reading and writing to range > > > > Process B > > ========= > > truncate(file_a) > > ... > > __break_lease() > > > > handle SIGIO from __break_lease > > if unhandled, process gets killed > > and put_user_pages should get called > > at some point here > > > > ...and so this way, user space gets to decide the proper behavior, > > instead of leaving the kernel in the dark with an impossible decision > > (kill process A? Block process B? User space knows the preference, > > per app, but kernel does not.) > > There is no need to kill anything here ... if truncate happens then > the GUP user is just GUPing page that do not correspond to anything > anymore. This is the current behavior and it is what GUP always has > been. By the time you get the page from GUP there is no garantee that > they correspond to anything. > > If a device really want to mirror process address faithfully then the > hardware need to make little effort either have something like ATS/ > PASID or be able to abide mmu notifier. > > If we start blocking existing syscall just because someone is doing a > GUP we are opening a pandora box. It is not just truncate, it is a > whole range of syscall that deals with either file or virtual address. > > The semantic of GUP is really the semantic of direct I/O and the > virtual address you are direct I/O-ing to/from and the rule there is: > do not do anything stupid to those virtual addresses while you are > doing direct I/O with them (no munmap, mremap, madvise, truncate, ...). > > > Same logic apply to file, when two process do thing to same file there > the kernel never get in the way of one process doing something the > other process did not expect. For instance one process mmaping the file > the other process truncating the file, if the first process try to access > the file through the mmap after the truncation it will get a sigbus. > > So i believe best we could do is send a SIGBUS to the process that has > GUPed a range of a file that is being truncated this would match what > we do for CPU acces. There is no reason access through GUP should be > handled any differently. I agree in sending SIGBUS but the fact is most "Process A"'s will not be handling SIGBUS and will then result in that process dying. Ira > > Cheers, > Jérôme >
