Re: [PATCH v6 5/6] nouveau: use new mmu interval notifiers

On 1/14/20 5:00 AM, Jason Gunthorpe wrote:

On Mon, Jan 13, 2020 at 02:47:02PM -0800, Ralph Campbell wrote:

...

void nouveau_svmm_fini(struct nouveau_svmm **psvmm) { struct nouveau_svmm *svmm = *psvmm;

• struct mmu_interval_notifier *mni;
• if (svmm) { mutex_lock(&svmm->mutex);

• while (true) {
  

• 	mni = mmu_interval_notifier_find(svmm->mm,
  

• 			&nouveau_svm_mni_ops, 0UL, ~0UL);
  

• 	if (!mni)
  

• 		break;
  

• 	mmu_interval_notifier_put(mni);
  

Oh, now I really don't like the name 'put'. It looks like mni is refcounted here, and it isn't. put should be called 'remove_deferred'

OK.

And then you also need a way to barrier this scheme on driver unload.

Good point. I can add something like void mmu_interval_notifier_synchronize(struct mm_struct *mm) that waits for deferred operations to complete similar to mmu_interval_read_begin().

...

• }
  

  svmm->vmm = NULL; mutex_unlock(&svmm->mutex);

• mmu_notifier_put(&svmm->notifier);
  

While here it was actually a refcount.

...

+static void nouveau_svmm_do_unmap(struct mmu_interval_notifier *mni,

• 		 const struct mmu_notifier_range *range)
  

+{

• struct svmm_interval *smi =

• container_of(mni, struct svmm_interval, notifier);
  

• struct nouveau_svmm *svmm = smi->svmm;
• unsigned long start = mmu_interval_notifier_start(mni);
• unsigned long last = mmu_interval_notifier_last(mni);

This whole algorithm only works if it is protected by the read side of the interval tree lock. Deserves at least a comment if not an assertion too.

This is called from the invalidate() callback and while holding the driver page table lock so the struct mmu_interval_notifier and the interval tree can't change. I will add comments for v7.

...

static int nouveau_range_fault(struct nouveau_svmm *svmm, struct nouveau_drm *drm, void *data, u32 size,

• 	       u64 *pfns, struct svm_notifier *notifier)
  

• 	       u64 *pfns, u64 start, u64 end)
  

  { unsigned long timeout = jiffies + msecs_to_jiffies(HMM_RANGE_DEFAULT_TIMEOUT); /* Have HMM fault pages within the fault window to the GPU. */ struct hmm_range range = {

• .notifier = &notifier->notifier,
  

• .start = notifier->notifier.interval_tree.start,
  

• .end = notifier->notifier.interval_tree.last + 1,
  

• .start = start,
  

• .end = end,
  

  .pfns = pfns, .flags = nouveau_svm_pfn_flags, .values = nouveau_svm_pfn_values,

• .default_flags = 0,
  

• .pfn_flags_mask = ~0UL,
  

  .pfn_shift = NVIF_VMM_PFNMAP_V0_ADDR_SHIFT, };

• struct mm_struct *mm = notifier->notifier.mm;

• struct mm_struct *mm = svmm->mm; long ret;

while (true) { if (time_after(jiffies, timeout)) return -EBUSY;

• range.notifier_seq = mmu_interval_read_begin(range.notifier);
  

• range.default_flags = 0;
  

• range.pfn_flags_mask = -1UL;
  

  down_read(&mm->mmap_sem);

mmap sem doesn't have to be held for the interval search, and again we have lifetime issues with the membership here.

I agree mmap_sem isn't needed for the interval search, it is needed if the search doesn't find a registered interval and one needs to be created to cover the underlying VMA. If an arbitrary size interval was created instead, then mmap_sem wouldn't be needed. I don't understand the lifetime/membership issue. The driver is the only thing that allocates, inserts, or removes struct mmu_interval_notifier and thus completely controls the lifetime.

...

• ret = nouveau_svmm_interval_find(svmm, &range);
  

• if (ret) {
  

• 	up_read(&mm->mmap_sem);
  

• 	return ret;
  

• }
  

• range.notifier_seq = mmu_interval_read_begin(range.notifier);
  

  ret = hmm_range_fault(&range, 0); up_read(&mm->mmap_sem); if (ret <= 0) {

I'm still not sure this is a better approach than what ODP does. It looks very expensive on the fault path..

Jason

ODP doesn't have this problem because users have to call ib_reg_mr() before any I/O can happen to the process address space. That is when mmu_interval_notifier_insert() / mmu_interval_notifier_remove() can be called and the driver doesn't have to worry about the interval changing sizes or being removed while I/O is happening. For GPU like devices, I'm trying to allow hardware access to any user level address without pre-registering it. That means inserting mmu interval notifiers for the ranges the GPU page faults on and updating the intervals as munmap() calls remove parts of the address space. I don't want to register an interval per page so the logical range is the underlying VMA.

It isn't that expensive, there is an extra driver lock/unlock as part of the lookup and possibly a find_vma() and kmalloc(GFP_ATOMIC) for new intervals. Also, the deferred interval updates for munmap(). Compared to the cost of updating PTEs in the device and GPU fault handling, this is minimal overhead.