Date:   Wed, 15 Jan 2020 14:09:47 -0800
From:   Ralph Campbell <rcampbell@...dia.com>
To:     Jason Gunthorpe <jgg@...lanox.com>
CC:     "linux-rdma@...r.kernel.org" <linux-rdma@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "nouveau@...ts.freedesktop.org" <nouveau@...ts.freedesktop.org>,
        "linux-kselftest@...r.kernel.org" <linux-kselftest@...r.kernel.org>,
        Jerome Glisse <jglisse@...hat.com>,
        "John Hubbard" <jhubbard@...dia.com>,
        Christoph Hellwig <hch@....de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Ben Skeggs <bskeggs@...hat.com>, Shuah Khan <shuah@...nel.org>
Subject: 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.

Powered by blists - more mailing lists