On Fri, Aug 24, 2018 at 07:54:19PM +0900, Tetsuo Handa wrote:

Two more worries for this patch.

[...]

quoted

--- 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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