Re: [PATCH v2 12/15] drm/amdgpu: Call find_vma under mmap_sem

["Christian König" <ckoenig.leichtzumerken@gmail.com>]

Am 29.10.19 um 17:28 schrieb Kuehling, Felix:
> On 2019-10-28 4:10 p.m., Jason Gunthorpe wrote:
>> From: Jason Gunthorpe <jgg@mellanox.com>
>>
>> find_vma() must be called under the mmap_sem, reorganize this code to
>> do the vma check after entering the lock.
>>
>> Further, fix the unlocked use of struct task_struct's mm, instead use
>> the mm from hmm_mirror which has an active mm_grab. Also the mm_grab
>> must be converted to a mm_get before acquiring mmap_sem or calling
>> find_vma().
>>
>> Fixes: 66c45500bfdc ("drm/amdgpu: use new HMM APIs and helpers")
>> Fixes: 0919195f2b0d ("drm/amdgpu: Enable amdgpu_ttm_tt_get_user_pages in worker threads")
>> Cc: Alex Deucher <alexander.deucher@amd.com>
>> Cc: Christian König <christian.koenig@amd.com>
>> Cc: David (ChunMing) Zhou <David1.Zhou@amd.com>
>> Cc: amd-gfx@lists.freedesktop.org
>> Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>
> One question inline to confirm my understanding. Otherwise this patch is
>
> Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
>
>
>> ---
>>    drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.c | 37 ++++++++++++++-----------
>>    1 file changed, 21 insertions(+), 16 deletions(-)
>>
>> diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.c
>> index dff41d0a85fe96..c0e41f1f0c2365 100644
>> --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.c
>> +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.c
>> @@ -35,6 +35,7 @@
>>    #include <linux/hmm.h>
>>    #include <linux/pagemap.h>
>>    #include <linux/sched/task.h>
>> +#include <linux/sched/mm.h>
>>    #include <linux/seq_file.h>
>>    #include <linux/slab.h>
>>    #include <linux/swap.h>
>> @@ -788,7 +789,7 @@ int amdgpu_ttm_tt_get_user_pages(struct amdgpu_bo *bo, struct page **pages)
>>    	struct hmm_mirror *mirror = bo->mn ? &bo->mn->mirror : NULL;
>>    	struct ttm_tt *ttm = bo->tbo.ttm;
>>    	struct amdgpu_ttm_tt *gtt = (void *)ttm;
>> -	struct mm_struct *mm = gtt->usertask->mm;
>> +	struct mm_struct *mm;
>>    	unsigned long start = gtt->userptr;
>>    	struct vm_area_struct *vma;
>>    	struct hmm_range *range;
>> @@ -796,25 +797,14 @@ int amdgpu_ttm_tt_get_user_pages(struct amdgpu_bo *bo, struct page **pages)
>>    	uint64_t *pfns;
>>    	int r = 0;
>>    
>> -	if (!mm) /* Happens during process shutdown */
>> -		return -ESRCH;
>> -
>>    	if (unlikely(!mirror)) {
>>    		DRM_DEBUG_DRIVER("Failed to get hmm_mirror\n");
>> -		r = -EFAULT;
>> -		goto out;
>> +		return -EFAULT;
>>    	}
>>    
>> -	vma = find_vma(mm, start);
>> -	if (unlikely(!vma || start < vma->vm_start)) {
>> -		r = -EFAULT;
>> -		goto out;
>> -	}
>> -	if (unlikely((gtt->userflags & AMDGPU_GEM_USERPTR_ANONONLY) &&
>> -		vma->vm_file)) {
>> -		r = -EPERM;
>> -		goto out;
>> -	}
>> +	mm = mirror->hmm->mmu_notifier.mm;
>> +	if (!mmget_not_zero(mm)) /* Happens during process shutdown */
> This works because mirror->hmm->mmu_notifier holds an mmgrab reference
> to the mm? So the MM will not just go away, but if the mmget refcount is
> 0, it means the mm is marked for destruction and shouldn't be used any more.

Yes, exactly. That is a rather common pattern, one reference count for 
the functionality and one for the structure.

When the functionality is gone the structure might still be alive for 
some reason. TTM and a couple of other structures use the same approach.

Christian.

>
>
>> +		return -ESRCH;
>>    
>>    	range = kzalloc(sizeof(*range), GFP_KERNEL);
>>    	if (unlikely(!range)) {
>> @@ -847,6 +837,17 @@ int amdgpu_ttm_tt_get_user_pages(struct amdgpu_bo *bo, struct page **pages)
>>    	hmm_range_wait_until_valid(range, HMM_RANGE_DEFAULT_TIMEOUT);
>>    
>>    	down_read(&mm->mmap_sem);
>> +	vma = find_vma(mm, start);
>> +	if (unlikely(!vma || start < vma->vm_start)) {
>> +		r = -EFAULT;
>> +		goto out_unlock;
>> +	}
>> +	if (unlikely((gtt->userflags & AMDGPU_GEM_USERPTR_ANONONLY) &&
>> +		vma->vm_file)) {
>> +		r = -EPERM;
>> +		goto out_unlock;
>> +	}
>> +
>>    	r = hmm_range_fault(range, 0);
>>    	up_read(&mm->mmap_sem);
>>    
>> @@ -865,15 +866,19 @@ int amdgpu_ttm_tt_get_user_pages(struct amdgpu_bo *bo, struct page **pages)
>>    	}
>>    
>>    	gtt->range = range;
>> +	mmput(mm);
>>    
>>    	return 0;
>>    
>> +out_unlock:
>> +	up_read(&mm->mmap_sem);
>>    out_free_pfns:
>>    	hmm_range_unregister(range);
>>    	kvfree(pfns);
>>    out_free_ranges:
>>    	kfree(range);
>>    out:
>> +	mmput(mm);
>>    	return r;
>>    }
>>    
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