[PATCH 2/3] drm/amdkfd: handle VMA remove race
Felix Kuehling
felix.kuehling at amd.com
Thu Nov 18 16:01:46 UTC 2021
Am 2021-11-18 um 10:55 a.m. schrieb philip yang:
>
>
> On 2021-11-18 10:07 a.m., Felix Kuehling wrote:
>> Am 2021-11-18 um 10:00 a.m. schrieb philip yang:
>>> On 2021-11-17 7:10 p.m., Felix Kuehling wrote:
>>>> On 2021-11-16 10:43 p.m., Philip Yang wrote:
>>>>> VMA may be removed before unmap notifier callback, restore pages take
>>>>> mmap write lock to lookup VMA to avoid race,
>>>> The old code looked up the VMA after taking the mmap lock (either
>>>> read or write) and kept holding the lock afterwards. I think even
>>>> with your new code it's possible that the VMA disappears before you
>>>> take the lock the first time, so always taking the write lock only
>>>> reduces the time window in which things can go wrong, but it doesn't
>>>> remove the race.
>>> Take mmap write lock will serialize with __do_munmap,
>> __do_munmap runs with the mmap write lock. Taking the read lock should
>> be sufficient to serialize with it.
>
> __do_munmap takes mmap write lock to remove vma, then downgrade to
> read lock to call unmap_region.
>
Yes. But it does that after detaching the VMA. So holding the read lock
is sufficient to ensure that a VMA you have looked up remains valid and
that __do_munmap will not remove it.
Regards,
Felix
> static int __vm_munmap(unsigned long start, size_t len, bool downgrade)
> {
> int ret;
> struct mm_struct *mm = current->mm;
> LIST_HEAD(uf);
>
> if (mmap_write_lock_killable(mm))
> return -EINTR;
>
> ret = __do_munmap(mm, start, len, &uf, downgrade);
> /*
> * Returning 1 indicates mmap_lock is downgraded.
> * But 1 is not legal return value of vm_munmap() and munmap(), reset
> * it to 0 before return.
> */
> if (ret == 1) {
> mmap_read_unlock(mm);
> ret = 0;
> } else
> mmap_write_unlock(mm);
>
> }
>
> int __do_munmap(struct mm_struct *mm, unsigned long start, size_t len,
> {
>
> ...
>
> /* Detach vmas from rbtree */
> if (!detach_vmas_to_be_unmapped(mm, vma, prev, end))
> downgrade = false;
>
> if (downgrade)
> mmap_write_downgrade(mm);
>
> unmap_region(mm, vma, prev, start, end);
>
> }
>
> Regards,
>
> Philip
>
>> Regards,
>> Felix
>>
>>
>>> to ensure vma remove and unmap callback are done, because unmap
>>> callback set drain_retryfaults flag, so we can safely drain the
>>> faults, and it is app bug if vma not found after taking mmap write lock.
>>>> I still struggle to understand the race you're trying to fix. The
>>>> only time the svm_restore_pages can see that the VMA is gone AND the
>>>> prange is gone is after the deferred worker has removed the prange.
>>>> But the fault draining in the deferred worker should prevent us from
>>>> ever seeing stale faults in that situation. That means, if no prange
>>>> is found and no VMA is found, it's definitely an application bug.
>>>>
>>>> The only possible race is in the case where the prange still exists
>>>> but the VMA is gone (needed by svm_fault_allowed). We can treat that
>>>> as a special case where we just return success because we know that
>>>> we're handling a stale fault for a VMA that's in the middle of being
>>>> unmapped. The fact that the prange still existed means that there
>>>> once was a valid mapping at the address but the deferred worker just
>>>> hasn't had a chance to clean it up yet.
>>>>
>>> Yes, that is only possible race.
>>>> One more comment inline.
>>>>
>>>>
>>>>> and then create unregister
>>>>> new range and check VMA access permission, then downgrade to take mmap
>>>>> read lock to recover fault. Refactor code to avoid duplicate VMA
>>>>> lookup.
>>>>>
>>>>> Signed-off-by: Philip Yang <Philip.Yang at amd.com>
>>>>> ---
>>>>> drivers/gpu/drm/amd/amdkfd/kfd_svm.c | 65
>>>>> ++++++++++------------------
>>>>> 1 file changed, 24 insertions(+), 41 deletions(-)
>>>>>
>>>>> diff --git a/drivers/gpu/drm/amd/amdkfd/kfd_svm.c
>>>>> b/drivers/gpu/drm/amd/amdkfd/kfd_svm.c
>>>>> index c1f367934428..3eb0a9491755 100644
>>>>> --- a/drivers/gpu/drm/amd/amdkfd/kfd_svm.c
>>>>> +++ b/drivers/gpu/drm/amd/amdkfd/kfd_svm.c
>>>>> @@ -2329,20 +2329,13 @@ svm_range_best_restore_location(struct
>>>>> svm_range *prange,
>>>>> }
>>>>> static int
>>>>> -svm_range_get_range_boundaries(struct kfd_process *p, int64_t addr,
>>>>> - unsigned long *start, unsigned long *last,
>>>>> - bool *is_heap_stack)
>>>>> +svm_range_get_range_boundaries(struct kfd_process *p, struct
>>>>> vm_area_struct *vma,
>>>>> + int64_t addr, unsigned long *start,
>>>>> + unsigned long *last, bool *is_heap_stack)
>>>>> {
>>>>> - struct vm_area_struct *vma;
>>>>> struct interval_tree_node *node;
>>>>> unsigned long start_limit, end_limit;
>>>>> - vma = find_vma(p->mm, addr << PAGE_SHIFT);
>>>>> - if (!vma || (addr << PAGE_SHIFT) < vma->vm_start) {
>>>>> - pr_debug("VMA does not exist in address [0x%llx]\n", addr);
>>>>> - return -EFAULT;
>>>>> - }
>>>>> -
>>>>> *is_heap_stack = (vma->vm_start <= vma->vm_mm->brk &&
>>>>> vma->vm_end >= vma->vm_mm->start_brk) ||
>>>>> (vma->vm_start <= vma->vm_mm->start_stack &&
>>>>> @@ -2437,9 +2430,10 @@ svm_range_check_vm_userptr(struct kfd_process
>>>>> *p, uint64_t start, uint64_t last,
>>>>> static struct
>>>>> svm_range *svm_range_create_unregistered_range(struct
>>>>> amdgpu_device *adev,
>>>>> - struct kfd_process *p,
>>>>> - struct mm_struct *mm,
>>>>> - int64_t addr)
>>>>> + struct kfd_process *p,
>>>>> + struct mm_struct *mm,
>>>>> + struct vm_area_struct *vma,
>>>>> + int64_t addr)
>>>>> {
>>>>> struct svm_range *prange = NULL;
>>>>> unsigned long start, last;
>>>>> @@ -2449,7 +2443,7 @@ svm_range
>>>>> *svm_range_create_unregistered_range(struct amdgpu_device *adev,
>>>>> uint64_t bo_l = 0;
>>>>> int r;
>>>>> - if (svm_range_get_range_boundaries(p, addr, &start, &last,
>>>>> + if (svm_range_get_range_boundaries(p, vma, addr, &start, &last,
>>>>> &is_heap_stack))
>>>>> return NULL;
>>>>> @@ -2552,20 +2546,13 @@ svm_range_count_fault(struct amdgpu_device
>>>>> *adev, struct kfd_process *p,
>>>>> }
>>>>> static bool
>>>>> -svm_fault_allowed(struct mm_struct *mm, uint64_t addr, bool
>>>>> write_fault)
>>>>> +svm_fault_allowed(struct vm_area_struct *vma, bool write_fault)
>>>>> {
>>>>> unsigned long requested = VM_READ;
>>>>> - struct vm_area_struct *vma;
>>>>> if (write_fault)
>>>>> requested |= VM_WRITE;
>>>>> - vma = find_vma(mm, addr << PAGE_SHIFT);
>>>>> - if (!vma || (addr << PAGE_SHIFT) < vma->vm_start) {
>>>>> - pr_debug("address 0x%llx VMA is removed\n", addr);
>>>>> - return true;
>>>>> - }
>>>>> -
>>>>> pr_debug("requested 0x%lx, vma permission flags 0x%lx\n",
>>>>> requested,
>>>>> vma->vm_flags);
>>>>> return (vma->vm_flags & requested) == requested;
>>>>> @@ -2582,7 +2569,7 @@ svm_range_restore_pages(struct amdgpu_device
>>>>> *adev, unsigned int pasid,
>>>>> uint64_t timestamp;
>>>>> int32_t best_loc;
>>>>> int32_t gpuidx = MAX_GPU_INSTANCE;
>>>>> - bool write_locked = false;
>>>>> + struct vm_area_struct *vma = NULL;
>>>>> int r = 0;
>>>>> if (!KFD_IS_SVM_API_SUPPORTED(adev->kfd.dev)) {
>>>>> @@ -2606,26 +2593,22 @@ svm_range_restore_pages(struct amdgpu_device
>>>>> *adev, unsigned int pasid,
>>>>> /* mm is available because kfd_process_notifier_release
>>>>> drain fault */
>>>>> mm = p->mm;
>>>>> + mmap_write_lock(mm);
>>>> Always taking the write lock is unnecessary. I think we can keep the
>>>> old strategy of retrying with the write lock only when necessary. I
>>>> think this should work correctly as long as you lookup the VMA every
>>>> time after taking either the mmap read or write lock. The vma you
>>>> looked up should be valid as long as you hold that lock.
>>>>
>>>> As I pointed out above, if svm_range_from_addr finds a prange but the
>>>> VMA is missing, we can treat that as a special case and return
>>>> success (just draining a stale fault on a VMA that's being unmapped).
>>> ok, I will change svm_fault_allowed to return success if VMA is
>>> missing, it is simpler to handle this special race case, without
>>> taking mmap write lock.
>>>
>>> Regards,
>>>
>>> Philip
>>>
>>>> Regards,
>>>> Felix
>>>>
>>>>
>>>>> +
>>>>> + vma = find_vma(p->mm, addr << PAGE_SHIFT);
>>>>> + if (!vma || (addr << PAGE_SHIFT) < vma->vm_start) {
>>>>> + pr_debug("VMA not found for address 0x%llx\n", addr);
>>>>> + mmap_write_downgrade(mm);
>>>>> + r = -EFAULT;
>>>>> + goto out_unlock_mm;
>>>>> + }
>>>>> - mmap_read_lock(mm);
>>>>> -retry_write_locked:
>>>>> mutex_lock(&svms->lock);
>>>>> prange = svm_range_from_addr(svms, addr, NULL);
>>>>> if (!prange) {
>>>>> pr_debug("failed to find prange svms 0x%p address
>>>>> [0x%llx]\n",
>>>>> svms, addr);
>>>>> - if (!write_locked) {
>>>>> - /* Need the write lock to create new range with MMU
>>>>> notifier.
>>>>> - * Also flush pending deferred work to make sure the
>>>>> interval
>>>>> - * tree is up to date before we add a new range
>>>>> - */
>>>>> - mutex_unlock(&svms->lock);
>>>>> - mmap_read_unlock(mm);
>>>>> - mmap_write_lock(mm);
>>>>> - write_locked = true;
>>>>> - goto retry_write_locked;
>>>>> - }
>>>>> - prange = svm_range_create_unregistered_range(adev, p, mm,
>>>>> addr);
>>>>> + prange = svm_range_create_unregistered_range(adev, p, mm,
>>>>> vma, addr);
>>>>> if (!prange) {
>>>>> pr_debug("failed to create unregistered range svms
>>>>> 0x%p address [0x%llx]\n",
>>>>> svms, addr);
>>>>> @@ -2634,8 +2617,8 @@ svm_range_restore_pages(struct amdgpu_device
>>>>> *adev, unsigned int pasid,
>>>>> goto out_unlock_svms;
>>>>> }
>>>>> }
>>>>> - if (write_locked)
>>>>> - mmap_write_downgrade(mm);
>>>>> +
>>>>> + mmap_write_downgrade(mm);
>>>>> mutex_lock(&prange->migrate_mutex);
>>>>> @@ -2652,7 +2635,7 @@ svm_range_restore_pages(struct amdgpu_device
>>>>> *adev, unsigned int pasid,
>>>>> goto out_unlock_range;
>>>>> }
>>>>> - if (!svm_fault_allowed(mm, addr, write_fault)) {
>>>>> + if (!svm_fault_allowed(vma, write_fault)) {
>>>>> pr_debug("fault addr 0x%llx no %s permission\n", addr,
>>>>> write_fault ? "write" : "read");
>>>>> r = -EPERM;
>>>>> @@ -2704,10 +2687,10 @@ svm_range_restore_pages(struct amdgpu_device
>>>>> *adev, unsigned int pasid,
>>>>> mutex_unlock(&prange->migrate_mutex);
>>>>> out_unlock_svms:
>>>>> mutex_unlock(&svms->lock);
>>>>> +out_unlock_mm:
>>>>> mmap_read_unlock(mm);
>>>>> svm_range_count_fault(adev, p, gpuidx);
>>>>> -
>>>>> out:
>>>>> kfd_unref_process(p);
>>>>>
More information about the amd-gfx
mailing list