Date:   Thu, 15 Aug 2019 13:35:11 -0400
From:   Jerome Glisse <jglisse@...hat.com>
To:     Daniel Vetter <daniel.vetter@...ll.ch>
Cc:     Jason Gunthorpe <jgg@...pe.ca>, Michal Hocko <mhocko@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>,
        DRI Development <dri-devel@...ts.freedesktop.org>,
        Intel Graphics Development <intel-gfx@...ts.freedesktop.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        David Rientjes <rientjes@...gle.com>,
        Christian König <christian.koenig@....com>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Wei Wang <wvw@...gle.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Jann Horn <jannh@...gle.com>, Feng Tang <feng.tang@...el.com>,
        Kees Cook <keescook@...omium.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        Daniel Vetter <daniel.vetter@...el.com>
Subject: Re: [PATCH 2/5] kernel.h: Add non_block_start/end()

On Thu, Aug 15, 2019 at 07:21:47PM +0200, Daniel Vetter wrote:
> On Thu, Aug 15, 2019 at 7:16 PM Jason Gunthorpe <jgg@...pe.ca> wrote:
> >
> > On Thu, Aug 15, 2019 at 12:32:38PM -0400, Jerome Glisse wrote:
> > > On Thu, Aug 15, 2019 at 12:10:28PM -0300, Jason Gunthorpe wrote:
> > > > On Thu, Aug 15, 2019 at 04:43:38PM +0200, Daniel Vetter wrote:
> > > >
> > > > > You have to wait for the gpu to finnish current processing in
> > > > > invalidate_range_start. Otherwise there's no point to any of this
> > > > > really. So the wait_event/dma_fence_wait are unavoidable really.
> > > >
> > > > I don't envy your task :|
> > > >
> > > > But, what you describe sure sounds like a 'registration cache' model,
> > > > not the 'shadow pte' model of coherency.
> > > >
> > > > The key difference is that a regirstationcache is allowed to become
> > > > incoherent with the VMA's because it holds page pins. It is a
> > > > programming bug in userspace to change VA mappings via mmap/munmap/etc
> > > > while the device is working on that VA, but it does not harm system
> > > > integrity because of the page pin.
> > > >
> > > > The cache ensures that each initiated operation sees a DMA setup that
> > > > matches the current VA map when the operation is initiated and allows
> > > > expensive device DMA setups to be re-used.
> > > >
> > > > A 'shadow pte' model (ie hmm) *really* needs device support to
> > > > directly block DMA access - ie trigger 'device page fault'. ie the
> > > > invalidate_start should inform the device to enter a fault mode and
> > > > that is it.  If the device can't do that, then the driver probably
> > > > shouldn't persue this level of coherency. The driver would quickly get
> > > > into the messy locking problems like dma_fence_wait from a notifier.
> > >
> > > I think here we do not agree on the hardware requirement. For GPU
> > > we will always need to be able to wait for some GPU fence from inside
> > > the notifier callback, there is just no way around that for many of
> > > the GPUs today (i do not see any indication of that changing).
> >
> > I didn't say you couldn't wait, I was trying to say that the wait
> > should only be contigent on the HW itself. Ie you can wait on a GPU
> > page table lock, and you can wait on a GPU page table flush completion
> > via IRQ.
> >
> > What is troubling is to wait till some other thread gets a GPU command
> > completion and decr's a kref on the DMA buffer - which kinda looks
> > like what this dma_fence() stuff is all about. A driver like that
> > would have to be super careful to ensure consistent forward progress
> > toward dma ref == 0 when the system is under reclaim.
> >
> > ie by running it's entire IRQ flow under fs_reclaim locking.
> 
> This is correct. At least for i915 it's already a required due to our
> shrinker also having to do the same. I think amdgpu isn't bothering
> with that since they have vram for most of the stuff, and just limit
> system memory usage to half of all and forgo the shrinker. Probably
> not the nicest approach. Anyway, both do the same mmu_notifier dance,
> just want to explain that we've been living with this for longer
> already.
> 
> So yeah writing a gpu driver is not easy.
> 
> > > associated with the mm_struct. In all GPU driver so far it is a short
> > > lived lock and nothing blocking is done while holding it (it is just
> > > about updating page table directory really wether it is filling it or
> > > clearing it).
> >
> > The main blocking I expect in a shadow PTE flow is waiting for the HW
> > to complete invalidations of its PTE cache.
> >
> > > > It is important to identify what model you are going for as defining a
> > > > 'registration cache' coherence expectation allows the driver to skip
> > > > blocking in invalidate_range_start. All it does is invalidate the
> > > > cache so that future operations pick up the new VA mapping.
> > > >
> > > > Intel's HFI RDMA driver uses this model extensively, and I think it is
> > > > well proven, within some limitations of course.
> > > >
> > > > At least, 'registration cache' is the only use model I know of where
> > > > it is acceptable to skip invalidate_range_end.
> > >
> > > Here GPU are not in the registration cache model, i know it might looks
> > > like it because of GUP but GUP was use just because hmm did not exist
> > > at the time.
> >
> > It is not because of GUP, it is because of the lack of
> > invalidate_range_end. A driver cannot correctly implement the SPTE
> > model without invalidate_range_end, even if it holds the page pins via
> > GUP.
> >
> > So, I've been assuming the few drivers without invalidate_range_end
> > are trying to do registration caching, rather than assuming they are
> > broken.
> 
> I915 might just be broken. amdgpu does the full thing, using
> hmm_mirror. But still with dma_fence_wait.

Yeah i915 is broken but it never hurted anyone ;) I posted patch
a long time ago to convert it to hmm but i delayed that to until
i can get through making something of GUPfast that can also be
use for HMM/ODP user.

Cheers,
Jérôme

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