Re: [PATCH] drm/amdgpu: use new HMM APIs and helpers v3

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On 2019-06-03 5:02 p.m., Kuehling, Felix wrote:
> On 2019-06-03 2:44 p.m., Yang, Philip wrote:
>> HMM provides new APIs and helps in kernel 5.2-rc1 to simplify driver
>> path. The old hmm APIs are deprecated and will be removed in future.
>>
>> Below are changes in driver:
>>
>> 1. Change hmm_vma_fault to hmm_range_register and hmm_range_fault which
>> supports range with multiple vmas, remove the multiple vmas handle path
>> and data structure.
>> 2. Change hmm_vma_range_done to hmm_range_unregister.
>> 3. Use default flags to avoid pre-fill pfn arrays.
>> 4. Use new hmm_device_ helpers.
>>
>> v2: retry if hmm_range_fault returns -EAGAIN
>> v3: define MAX_RETRY_HMM_RANGE_FAULT, skip drop mmap_sem if retry
> 
> I think dropping mmap_sem for retry was correct in v2 of this patch. Now
> you will try to take the mmap_sem multiple times without dropping it if
> you retry.
> 
> We don't need to hold the mmap_sem during hmm_range_wait_until_valid.
> 
hmm_range_fault will drop the mmap_sem internally before returning 
-EAGAIN, I think it does this on purpose to avoid upper level call retry 
whithout release mmap_sem in between. I find v2 was incorrect after 
reading hmm code again today. I should mention details of this change in 
my commit, see more below.

Philip

> See more below ...
> 
> 
>>
>> Change-Id: I1a00f88459f3b96c9536933e9a17eb1ebaa78113
>> Signed-off-by: Philip Yang <Philip.Yang@xxxxxxx>
>> ---
>>    drivers/gpu/drm/amd/amdgpu/amdgpu_mn.c  |   1 -
>>    drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.c | 149 ++++++++++--------------
>>    2 files changed, 64 insertions(+), 86 deletions(-)
>>
>> diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_mn.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_mn.c
>> index 41ccee49a224..e0bb47ce570b 100644
>> --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_mn.c
>> +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_mn.c
>> @@ -519,7 +519,6 @@ void amdgpu_hmm_init_range(struct hmm_range *range)
>>    		range->flags = hmm_range_flags;
>>    		range->values = hmm_range_values;
>>    		range->pfn_shift = PAGE_SHIFT;
>> -		range->pfns = NULL;
>>    		INIT_LIST_HEAD(&range->list);
>>    	}
>>    }
>> diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.c
>> index eb4b85061c7e..9de6a2f5e572 100644
>> --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.c
>> +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.c
>> @@ -711,8 +711,7 @@ struct amdgpu_ttm_tt {
>>    	struct task_struct	*usertask;
>>    	uint32_t		userflags;
>>    #if IS_ENABLED(CONFIG_DRM_AMDGPU_USERPTR)
>> -	struct hmm_range	*ranges;
>> -	int			nr_ranges;
>> +	struct hmm_range	*range;
>>    #endif
>>    };
>>    
>> @@ -725,57 +724,36 @@ struct amdgpu_ttm_tt {
>>     */
>>    #if IS_ENABLED(CONFIG_DRM_AMDGPU_USERPTR)
>>    
>> -/* Support Userptr pages cross max 16 vmas */
>> -#define MAX_NR_VMAS	(16)
>> +#define MAX_RETRY_HMM_RANGE_FAULT	16
>>    
>>    int amdgpu_ttm_tt_get_user_pages(struct ttm_tt *ttm, struct page **pages)
>>    {
>>    	struct amdgpu_ttm_tt *gtt = (void *)ttm;
>>    	struct mm_struct *mm = gtt->usertask->mm;
>>    	unsigned long start = gtt->userptr;
>> -	unsigned long end = start + ttm->num_pages * PAGE_SIZE;
>> -	struct vm_area_struct *vma = NULL, *vmas[MAX_NR_VMAS];
>> -	struct hmm_range *ranges;
>> -	unsigned long nr_pages, i;
>> -	uint64_t *pfns, f;
>> +	struct vm_area_struct *vma;
>> +	struct hmm_range *range;
>> +	unsigned long i;
>> +	uint64_t *pfns;
>> +	int retry = 0;
>>    	int r = 0;
>>    
>>    	if (!mm) /* Happens during process shutdown */
>>    		return -ESRCH;
>>    
>> -	down_read(&mm->mmap_sem);
>> -
>> -	/* user pages may cross multiple VMAs */
>> -	gtt->nr_ranges = 0;
>> -	do {
>> -		unsigned long vm_start;
>> -
>> -		if (gtt->nr_ranges >= MAX_NR_VMAS) {
>> -			DRM_ERROR("Too many VMAs in userptr range\n");
>> -			r = -EFAULT;
>> -			goto out;
>> -		}
>> -
>> -		vm_start = vma ? vma->vm_end : start;
>> -		vma = find_vma(mm, vm_start);
>> -		if (unlikely(!vma || vm_start < vma->vm_start)) {
>> -			r = -EFAULT;
>> -			goto out;
>> -		}
>> -		vmas[gtt->nr_ranges++] = vma;
>> -	} while (end > vma->vm_end);
>> -
>> -	DRM_DEBUG_DRIVER("0x%lx nr_ranges %d pages 0x%lx\n",
>> -		start, gtt->nr_ranges, ttm->num_pages);
>> -
>> +	vma = find_vma(mm, start);
>> +	if (unlikely(!vma || start < vma->vm_start)) {
>> +		r = -EFAULT;
>> +		goto out;
>> +	}
>>    	if (unlikely((gtt->userflags & AMDGPU_GEM_USERPTR_ANONONLY) &&
>> -		vmas[0]->vm_file)) {
>> +		vma->vm_file)) {
>>    		r = -EPERM;
>>    		goto out;
>>    	}
>>    
>> -	ranges = kvmalloc_array(gtt->nr_ranges, sizeof(*ranges), GFP_KERNEL);
>> -	if (unlikely(!ranges)) {
>> +	range = kzalloc(sizeof(*range), GFP_KERNEL);
>> +	if (unlikely(!range)) {
>>    		r = -ENOMEM;
>>    		goto out;
>>    	}
>> @@ -786,61 +764,62 @@ int amdgpu_ttm_tt_get_user_pages(struct ttm_tt *ttm, struct page **pages)
>>    		goto out_free_ranges;
>>    	}
>>    
>> -	for (i = 0; i < gtt->nr_ranges; i++)
>> -		amdgpu_hmm_init_range(&ranges[i]);
>> +	amdgpu_hmm_init_range(range);
>> +	range->default_flags = range->flags[HMM_PFN_VALID];
>> +	range->default_flags |= amdgpu_ttm_tt_is_readonly(ttm) ?
>> +				0 : range->flags[HMM_PFN_WRITE];
>> +	range->pfn_flags_mask = 0;
>> +	range->pfns = pfns;
>> +	hmm_range_register(range, mm, start,
>> +			   start + ttm->num_pages * PAGE_SIZE, PAGE_SHIFT);
>>    
>> -	f = ranges[0].flags[HMM_PFN_VALID];
>> -	f |= amdgpu_ttm_tt_is_readonly(ttm) ?
>> -				0 : ranges[0].flags[HMM_PFN_WRITE];
>> -	memset64(pfns, f, ttm->num_pages);
>> +retry:
>> +	/*
>> +	 * Just wait for range to be valid, safe to ignore return value as we
>> +	 * will use the return value of hmm_range_fault() below under the
>> +	 * mmap_sem to ascertain the validity of the range.
>> +	 */
>> +	hmm_range_wait_until_valid(range, HMM_RANGE_DEFAULT_TIMEOUT);
>>    
>> -	for (nr_pages = 0, i = 0; i < gtt->nr_ranges; i++) {
>> -		ranges[i].vma = vmas[i];
>> -		ranges[i].start = max(start, vmas[i]->vm_start);
>> -		ranges[i].end = min(end, vmas[i]->vm_end);
>> -		ranges[i].pfns = pfns + nr_pages;
>> -		nr_pages += (ranges[i].end - ranges[i].start) / PAGE_SIZE;
>> +	down_read(&mm->mmap_sem);
> 
> If you take the mmap_sem here, you have to drop it before goto retry.
> Otherwise you'll end up taking the mmap_sem multiple times. It's not
> fatal immediately because you can take multiple read locks. But if you
> don't drop all the read locks, no writer will ever be able to take a
> write lock again.
> 
mmap_sem dropped by hmm_range_fault before returning -EAGAIN, have to 
take it again.
> 
>>    
>> -		r = hmm_vma_fault(&ranges[i], true);
>> -		if (unlikely(r))
>> -			break;
>> -	}
>> -	if (unlikely(r)) {
>> -		while (i--)
>> -			hmm_vma_range_done(&ranges[i]);
>> +	r = hmm_range_fault(range, true);
>> +	if (unlikely(r < 0)) {
>> +		if (likely(r == -EAGAIN)) {
>> +			if (retry++ < MAX_RETRY_HMM_RANGE_FAULT)
>> +				goto retry;
> 
> You're still holding the mmap_sem here.
> 
> 
>> +			else
>> +				pr_err("Retry hmm fault too many times\n");
>> +		}
>>    
>>    		goto out_free_pfns;
>>    	}
>>    
>> -	up_read(&mm->mmap_sem);
>> -
>>    	for (i = 0; i < ttm->num_pages; i++) {
>> -		pages[i] = hmm_pfn_to_page(&ranges[0], pfns[i]);
>> -		if (!pages[i]) {
>> +		pages[i] = hmm_device_entry_to_page(range, pfns[i]);
>> +		if (unlikely(!pages[i])) {
>>    			pr_err("Page fault failed for pfn[%lu] = 0x%llx\n",
>>    			       i, pfns[i]);
>> -			goto out_invalid_pfn;
>> +			r = -ENOMEM;
>> +
>> +			goto out_free_pfns;
>>    		}
>>    	}
>> -	gtt->ranges = ranges;
>> +
>> +	up_read(&mm->mmap_sem);
> 
> Why do you drop the mmap_sem so late? Does the loop above require it? I
> don't believe it does.
> 
Yes, I can drop the mmap_sem before the loop, just wanted to remove one 
line of duplicate up_read call.

> Regards,
>     Felix
> 
> 
>> +
>> +	gtt->range = range;
>>    
>>    	return 0;
>>    
>>    out_free_pfns:
>> +	up_read(&mm->mmap_sem);
>> +	hmm_range_unregister(range);
>>    	kvfree(pfns);
>>    out_free_ranges:
>> -	kvfree(ranges);
>> +	kfree(range);
>>    out:
>> -	up_read(&mm->mmap_sem);
>> -
>>    	return r;
>> -
>> -out_invalid_pfn:
>> -	for (i = 0; i < gtt->nr_ranges; i++)
>> -		hmm_vma_range_done(&ranges[i]);
>> -	kvfree(pfns);
>> -	kvfree(ranges);
>> -	return -ENOMEM;
>>    }
>>    
>>    /**
>> @@ -853,23 +832,23 @@ bool amdgpu_ttm_tt_get_user_pages_done(struct ttm_tt *ttm)
>>    {
>>    	struct amdgpu_ttm_tt *gtt = (void *)ttm;
>>    	bool r = false;
>> -	int i;
>>    
>>    	if (!gtt || !gtt->userptr)
>>    		return false;
>>    
>> -	DRM_DEBUG_DRIVER("user_pages_done 0x%llx nr_ranges %d pages 0x%lx\n",
>> -		gtt->userptr, gtt->nr_ranges, ttm->num_pages);
>> +	DRM_DEBUG_DRIVER("user_pages_done 0x%llx pages 0x%lx\n",
>> +		gtt->userptr, ttm->num_pages);
>>    
>> -	WARN_ONCE(!gtt->ranges || !gtt->ranges[0].pfns,
>> +	WARN_ONCE(!gtt->range || !gtt->range->pfns,
>>    		"No user pages to check\n");
>>    
>> -	if (gtt->ranges) {
>> -		for (i = 0; i < gtt->nr_ranges; i++)
>> -			r |= hmm_vma_range_done(&gtt->ranges[i]);
>> -		kvfree(gtt->ranges[0].pfns);
>> -		kvfree(gtt->ranges);
>> -		gtt->ranges = NULL;
>> +	if (gtt->range) {
>> +		r = hmm_range_valid(gtt->range);
>> +		hmm_range_unregister(gtt->range);
>> +
>> +		kvfree(gtt->range->pfns);
>> +		kfree(gtt->range);
>> +		gtt->range = NULL;
>>    	}
>>    
>>    	return r;
>> @@ -953,9 +932,9 @@ static void amdgpu_ttm_tt_unpin_userptr(struct ttm_tt *ttm)
>>    	sg_free_table(ttm->sg);
>>    
>>    #if IS_ENABLED(CONFIG_DRM_AMDGPU_USERPTR)
>> -	if (gtt->ranges &&
>> -	    ttm->pages[0] == hmm_pfn_to_page(&gtt->ranges[0],
>> -					     gtt->ranges[0].pfns[0]))
>> +	if (gtt->range &&
>> +	    ttm->pages[0] == hmm_device_entry_to_page(gtt->range,
>> +						      gtt->range->pfns[0]))
>>    		WARN_ONCE(1, "Missing get_user_page_done\n");
>>    #endif
>>    }
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