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

Tetsuo Handa
2018-08-24T10:54:19Z

Two more worries for this patch.

> --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_mn.c
> +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_mn.c
> @@ -178,12 +178,18 @@ void amdgpu_mn_unlock(struct amdgpu_mn *mn)
> *
> * @amn: our notifier
> */
> -static void amdgpu_mn_read_lock(struct amdgpu_mn *amn)
> +static int amdgpu_mn_read_lock(struct amdgpu_mn *amn, bool blockable)
> {
> - mutex_lock(&amn->read_lock);
> + if (blockable)
> + mutex_lock(&amn->read_lock);
> + else if (!mutex_trylock(&amn->read_lock))
> + return -EAGAIN;
> +
> if (atomic_inc_return(&amn->recursion) == 1)
> down_read_non_owner(&amn->lock);

Why don't we need to use trylock here if blockable == false ?
Want comment why it is safe to use blocking lock here.

> mutex_unlock(&amn->read_lock);
> +
> + return 0;
> }
>
> /**

> --- 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.