Re: [Xen-devel] [PATCH] mm, oom: distinguish blockable mode for mmu notifiers

[Jerome Glisse <jglisse@xxxxxxxxxx>]

On Fri, Aug 24, 2018 at 07:54:19PM +0900, Tetsuo Handa wrote:
> Two more worries for this patch.

[...]

> 
> > --- a/mm/hmm.c
> > +++ b/mm/hmm.c
> > @@ -177,16 +177,19 @@ static void hmm_release(struct mmu_notifier *mn, 
> > struct mm_struct *mm)
> >         up_write(&hmm->mirrors_sem);
> >  }
> > 
> > -static void hmm_invalidate_range_start(struct mmu_notifier *mn,
> > +static int hmm_invalidate_range_start(struct mmu_notifier *mn,
> >                                        struct mm_struct *mm,
> >                                        unsigned long start,
> > -                                      unsigned long end)
> > +                                      unsigned long end,
> > +                                      bool blockable)
> >  {
> >         struct hmm *hmm = mm->hmm;
> > 
> >         VM_BUG_ON(!hmm);
> > 
> >         atomic_inc(&hmm->sequence);
> > +
> > +       return 0;
> >  }
> > 
> >  static void hmm_invalidate_range_end(struct mmu_notifier *mn,
> 
> This assumes that hmm_invalidate_range_end() does not have memory
> allocation dependency. But hmm_invalidate_range() from
> hmm_invalidate_range_end() involves
> 
>         down_read(&hmm->mirrors_sem);
>         list_for_each_entry(mirror, &hmm->mirrors, list)
>                 mirror->ops->sync_cpu_device_pagetables(mirror, action,
>                                                         start, end);
>         up_read(&hmm->mirrors_sem);
> 
> sequence. What is surprising is that there is no in-tree user who assigns
> sync_cpu_device_pagetables field.
> 
>   $ grep -Fr sync_cpu_device_pagetables *
>   Documentation/vm/hmm.rst:     /* sync_cpu_device_pagetables() - synchronize 
> page tables
>   include/linux/hmm.h: * will get callbacks through 
> sync_cpu_device_pagetables() operation (see
>   include/linux/hmm.h:    /* sync_cpu_device_pagetables() - synchronize page 
> tables
>   include/linux/hmm.h:    void (*sync_cpu_device_pagetables)(struct 
> hmm_mirror *mirror,
>   include/linux/hmm.h: * hmm_mirror_ops.sync_cpu_device_pagetables() 
> callback, so that CPU page
>   mm/hmm.c:               mirror->ops->sync_cpu_device_pagetables(mirror, 
> action,
> 
> That is, this API seems to be currently used by only out-of-tree users. Since
> we can't check that nobody has memory allocation dependency, I think that
> hmm_invalidate_range_start() should return -EAGAIN if blockable == false for 
> now.

So you can see update and user of this there:

https://cgit.freedesktop.org/~glisse/linux/log/?h=hmm-intel-v00
https://cgit.freedesktop.org/~glisse/linux/log/?h=hmm-nouveau-v01
https://cgit.freedesktop.org/~glisse/linux/log/?h=hmm-radeon-v00

I am still working on Mellanox and AMD GPU patchset.

I will post the HMM changes that adapt to Michal shortly as anyway
thus have been sufficiently tested by now.

https://cgit.freedesktop.org/~glisse/linux/commit/?h=hmm-4.20&id=78785dcb5ba0924c2c5e7be027793f99ebbc39f3
https://cgit.freedesktop.org/~glisse/linux/commit/?h=hmm-4.20&id=4fc25571dc893f2b278e90cda9e71e139e01de70

Cheers,
Jérôme

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