Re: [PATCH 1/2] mm: introduce put_user_page*(), placeholder versions

[Jerome Glisse]

On Thu, Dec 13, 2018 at 09:51:18AM -0500, Tom Talpey wrote:
> On 12/13/2018 9:18 AM, Jerome Glisse wrote:
> > On Thu, Dec 13, 2018 at 08:40:49AM -0500, Tom Talpey wrote:
> > > On 12/13/2018 7:43 AM, Jerome Glisse wrote:
> > > > On Wed, Dec 12, 2018 at 08:20:43PM -0700, Jason Gunthorpe wrote:
> > > > > On Wed, Dec 12, 2018 at 07:01:09PM -0500, Jerome Glisse wrote:
> > > > > > On Wed, Dec 12, 2018 at 04:37:03PM -0700, Jason Gunthorpe wrote:
> > > > > > > On Wed, Dec 12, 2018 at 04:53:49PM -0500, Jerome Glisse wrote:
> > > > > > > > > Almost, we need some safety around assuming that DMA is complete the
> > > > > > > > > page, so the notification would need to go all to way to userspace
> > > > > > > > > with something like a file lease notification. It would also need to
> > > > > > > > > be backstopped by an IOMMU in the case where the hardware does not /
> > > > > > > > > can not stop in-flight DMA.
> > > > > > > > 
> > > > > > > > You can always reprogram the hardware right away it will redirect
> > > > > > > > any dma to the crappy page.
> > > > > > > 
> > > > > > > That causes silent data corruption for RDMA users - we can't do that.
> > > > > > > 
> > > > > > > The only way out for current hardware is to forcibly terminate the
> > > > > > > RDMA activity somehow (and I'm not even sure this is possible, at
> > > > > > > least it would be driver specific)
> > > > > > > 
> > > > > > > Even the IOMMU idea probably doesn't work, I doubt all current
> > > > > > > hardware can handle a PCI-E error TLP properly.
> > > > > > 
> > > > > > What i saying is reprogram hardware to crappy page ie valid page
> > > > > > dma map but that just has random content as a last resort to allow
> > > > > > filesystem to reuse block. So their should be no PCIE error unless
> > > > > > hardware freak out to see its page table reprogram randomly.
> > > > > 
> > > > > No, that isn't an option. You can't silently provide corrupted data
> > > > > for RDMA to transfer out onto the network, or silently discard data
> > > > > coming in!!
> > > > > 
> > > > > Think of the consequences of that - I have a fileserver process and
> > > > > someone does ftruncate and now my clients receive corrupted data??
> > > > 
> > > > This is what happens _today_ ie today someone do GUP on page file
> > > > and then someone else do truncate the first GUP is effectively
> > > > streaming _random_ data to network as the page does not correspond
> > > > to anything anymore and once the RDMA MR goes aways and release
> > > > the page the page content will be lost. So i am not changing anything
> > > > here, what i proposed was to make it explicit to device driver at
> > > > least that they were streaming random data. Right now this is all
> > > > silent but this is what is happening wether you like it or not :)
> > > > 
> > > > Note that  i am saying do that only for truncate to allow to be
> > > > nice to fs. But again i am fine with whatever solution but you can
> > > > not please everyone here. Either block truncate and fs folks will
> > > > hate you or make it clear to device driver that you are streaming
> > > > random things and RDMA people hates you.
> > > > 
> > > > 
> > > > > The only option is to prevent the RDMA transfer from ever happening,
> > > > > and we just don't have hardware support (beyond destroy everything) to
> > > > > do that.
> > > > > 
> > > > > > The question is who do you want to punish ? RDMA user that pin stuff
> > > > > > and expect thing to work forever without worrying for other fs
> > > > > > activities ? Or filesystem to pin block forever :)
> > > > > 
> > > > > I don't want to punish everyone, I want both sides to have complete
> > > > > data integrity as the USER has deliberately decided to combine DAX and
> > > > > RDMA. So either stop it at the front end (ie get_user_pages_longterm)
> > > > > or make it work in a way that guarantees integrity for both.
> > > > > 
> > > > > >       S2: notify userspace program through device/sub-system
> > > > > >           specific API and delay ftruncate. After a while if there
> > > > > >           is no answer just be mean and force hardware to use
> > > > > >           crappy page as anyway this is what happens today
> > > > > 
> > > > > I don't think this happens today (outside of DAX).. Does it?
> > > > 
> > > > It does it is just silent, i don't remember anything in the code
> > > > that would stop a truncate to happen because of elevated refcount.
> > > > This does not happen with ODP mlx5 as it does abide by _all_ mmu
> > > > notifier. This is for anything that does ODP without support for
> > > > mmu notifier.
> > > 
> > > Wait - is it expected that the MMU notifier upcall is handled
> > > synchronously? That is, the page DMA mapping must be torn down
> > > immediately, and before returning?
> > 
> > Yes you must torn down mapping before returning from mmu notifier
> > call back. Any time after is too late. You obviously need hardware
> > that can support that. In the infiniband sub-system AFAIK only the
> > mlx5 hardware can do that. In the GPU sub-system everyone is fine.
> 
> I'm skeptical that MLX5 can actually make this guarantee. But we
> can take that offline in linux-rdma.

It does unless the code lies about what the hardware do :) See umem_odp.c
in core and odp.c in mlx5 directories.


> I'm also skeptical that NVMe can do this.
> 
> > Dunno about other sub-systems.
> > 
> > 
> > > That's simply not possible, since the hardware needs to get control
> > > to do this. Even if there were an IOMMU that could intercept the
> > > DMA, reprogramming it will require a flush, which cannot be guaranteed
> > > to occur "inline".
> > 
> > If hardware can not do that then hardware should not use GUP, at
> > least not on file back page. I advocated in favor of forbiding GUP
> > for device that can not do that as right now this silently breaks
> > in few cases (truncate, mremap, splice, reflink, ...). So the device
> > in those cases can end up with GUPed pages that do not correspond
> > to anything anymore ie they do not correspond to the memory backing
> > the virtual address they were GUP against, nor they correspond to
> > the file content at the given offset anymore. It is just random
> > data as far as the kernel or filesystem is concern.
> > 
> > Of course for this to happen you need an application that do stupid
> > thing like create an MR in one thread on the mmap of a file and
> > truncate that same file in another thread (or from the same thread).
> > 
> > So this is unlikely to happen in sane program. It does not mean it
> > will not happen.
> 
> Completely agree. In other words, this is the responsibility of the
> DAX (or g-u-p) consumer, which is nt necessarily the program itself,
> it could be an upper layer.
> 
> In SMB3 and NFSv4, which I've been focused on, we envision using the
> existing protocol leases to protect this. When requesting a DAX mapping,
> the server may requires an exclusive lease. If this mapping needs to
> change, because of another conflicting mapping, the lease would be
> recalled and the mapping dropped. This is a normal and well-established
> filesystem requirement.
> 
> The twist here is that the platform itself can initiate such an event.
> It's my belief that this plumbing must flow to the *top* of the stack,
> i.e. the entity that took the mapping (e.g. filesystem), and not
> depend on the MMU notifier at the very bottom.

So this patchset is about mm plumbings, what fs does before mm code
gets call is a fs discussion. Note also that GUP is always against
a virtual address of a process. GUP is use by few driver to allow
the device direct access to some portion of process address space.

The issues is that some of those device have designed an API that
fully ignore things like munmap, splice, truncate, ... and as such
they open the door for undefined behavior. I believe the original
intention in all the cases was that the user would not do stupid
thing like setup the device mapping through device specific API and
then munmap or truncate or anything that would affect the range of
virtual address.

Thing is from kernel point of view we should not and can not assume
that userspace will behave properly. So we have to brace for the
worst. Which is what this patchset is trying to do ie fix some of
issues and make the rest of them explicit to device driver so that
they can decide what to do about it.

My advice is for each of those sub-system/device to fail loudly
when such thing happens so that the user knows he is doing something
stupid or illegal.


> > The second set of issue at to deals with set_page_dirty happening
> > long time after page_release did happens and thus the fs dirty
> > page callback will see page in bad state and will BUG() and you
> > will have an oops and loose any data your device might have written
> > to the page. This is highly filesystem dependend and also timing
> > dependend and link to thing like memory pressure so it might not
> > happen that often but again it can happen.
> > 
> > 
> > > > > .. and the remedy here is to kill the process, not provide corrupt
> > > > > data. Kill the process is likely to not go over well with any real
> > > > > users that want this combination.
> > > > > 
> > > > > Think Samba serving files over RDMA - you can't have random unpriv
> > > > > users calling ftruncate and causing smbd to be killed or serve corrupt
> > > > > data.
> > > > 
> > > > So what i am saying is there is a choice and it would be better to
> > > > decide something than let the existing status quo where we just keep
> > > > streaming random data after truncate to a GUPed page.
> > > 
> > > Let's also remember that any torn-down DMA mapping can't be recycled
> > > until all uses of the old DMA addresses are destroyed. The whole
> > > thing screams for reference counting all the way down, to me.
> > 
> > I am not saying reuse the DMA address in the emergency_mean_callback
> > the idea was:
> > 
> >      gup_page_emergency_revoke(device, page)
> >      {
> >          crapy_page = alloc_page();
> >          dma_addr = dma_map(crappy_page, device, ...);
> >          mydevice_page_table_update(device, crappy_page, dma_addr);
> >          mydevice_tlb_flush(device);
> >          mydevice_wait_pending_dma(device)
> > 
> >          // at this point the original GUPed page is not access by hw
> > 
> >          dma_unmap(page);
> >          put_user_page(page);
> >      }
> 
> Ok, but my concern was also that the old DMA address then becomes
> unused and may be grabbed by a new i/o. If the hardware still has
> reads or writes in flight, and they arrive after the old address
> becomes valid, well, oops.

In above code at the comment point the driver garanty the hardware
will never use the old dma address any more ie that all in flight
dma are done. I know this can be done for some devices. It might
very well not work for all and this is why this need to be a sub-
system/device discussion.

Cheers,
Jérôme