lkml.org 
[lkml]   [2020]   [Jan]   [15]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
SubjectRe: [PATCH v6 5/6] nouveau: use new mmu interval notifiers
From
Date

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.

\
 
 \ /
  Last update: 2020-01-15 23:10    [W:0.116 / U:7.088 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site