Re: [PATCH 2/4] mm/hugeltb: simplify the return code of __vma_reservation_common()

[Miaohe Lin <linmiaohe@huawei.com>]

On 2021/4/7 10:37, Mike Kravetz wrote:
> On 4/6/21 7:05 PM, Miaohe Lin wrote:
>> Hi:
>> On 2021/4/7 8:53, Mike Kravetz wrote:
>>> On 4/2/21 2:32 AM, Miaohe Lin wrote:
>>>> It's guaranteed that the vma is associated with a resv_map, i.e. either
>>>> VM_MAYSHARE or HPAGE_RESV_OWNER, when the code reaches here or we would
>>>> have returned via !resv check above. So ret must be less than 0 in the
>>>> 'else' case. Simplify the return code to make this clear.
>>>
>>> I believe we still neeed that ternary operator in the return statement.
>>> Why?
>>>
>>> There are two basic types of mappings to be concerned with:
>>> shared and private.
>>> For private mappings, a task can 'own' the mapping as indicated by
>>> HPAGE_RESV_OWNER.  Or, it may not own the mapping.  The most common way
>>> to create a non-owner private mapping is to have a task with a private
>>> mapping fork.  The parent process will have HPAGE_RESV_OWNER set, the
>>> child process will not.  The idea is that since the child has a COW copy
>>> of the mapping it should not consume reservations made by the parent.
>>
>> The child process will not have HPAGE_RESV_OWNER set because at fork time, we do:
>> 		/*
>> 		 * Clear hugetlb-related page reserves for children. This only
>> 		 * affects MAP_PRIVATE mappings. Faults generated by the child
>> 		 * are not guaranteed to succeed, even if read-only
>> 		 */
>> 		if (is_vm_hugetlb_page(tmp))
>> 			reset_vma_resv_huge_pages(tmp);
>> i.e. we have vma->vm_private_data = (void *)0; for child process and vma_resv_map() will
>> return NULL in this case.
>> Or am I missed something?
>>
>>> Only the parent (HPAGE_RESV_OWNER) is allowed to consume the
>>> reservations.
>>> Hope that makens sense?
>>>
>>>>
>>>> Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
>>>> ---
>>>>  mm/hugetlb.c | 2 +-
>>>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>>>
>>>> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
>>>> index a03a50b7c410..b7864abded3d 100644
>>>> --- a/mm/hugetlb.c
>>>> +++ b/mm/hugetlb.c
>>>> @@ -2183,7 +2183,7 @@ static long __vma_reservation_common(struct hstate *h,
>>>>  			return 1;
>>>>  	}
>>>>  	else
>>>
>>> This else also handles the case !HPAGE_RESV_OWNER.  In this case, we
>>
>> IMO, for the case !HPAGE_RESV_OWNER, we won't reach here. What do you think?
>>
> 
> I think you are correct.
> 
> However, if this is true we should be able to simply the code even
> further.  There is no need to check for HPAGE_RESV_OWNER because we know
> it must be set.  Correct?  If so, the code could look something like:
> 
> 	if (vma->vm_flags & VM_MAYSHARE)
> 		return ret;
> 
> 	/* We know private mapping with HPAGE_RESV_OWNER */
> 	 * ...						 *
> 	 * Add that existing comment                     */
> 
> 	if (ret > 0)
> 		return 0;
> 	if (ret == 0)
> 		return 1;
> 	return ret;
> 

Many thanks for good suggestion! What do you mean is this ?

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index a03a50b7c410..9b4c05699a90 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -2163,27 +2163,26 @@ static long __vma_reservation_common(struct hstate *h,

        if (vma->vm_flags & VM_MAYSHARE)
                return ret;
-       else if (is_vma_resv_set(vma, HPAGE_RESV_OWNER) && ret >= 0) {
-               /*
-                * In most cases, reserves always exist for private mappings.
-                * However, a file associated with mapping could have been
-                * hole punched or truncated after reserves were consumed.
-                * As subsequent fault on such a range will not use reserves.
-                * Subtle - The reserve map for private mappings has the
-                * opposite meaning than that of shared mappings.  If NO
-                * entry is in the reserve map, it means a reservation exists.
-                * If an entry exists in the reserve map, it means the
-                * reservation has already been consumed.  As a result, the
-                * return value of this routine is the opposite of the
-                * value returned from reserve map manipulation routines above.
-                */
-               if (ret)
-                       return 0;
-               else
-                       return 1;
-       }
-       else
-               return ret < 0 ? ret : 0;
+       /*
+        * We know private mapping must have HPAGE_RESV_OWNER set.
+        *
+        * In most cases, reserves always exist for private mappings.
+        * However, a file associated with mapping could have been
+        * hole punched or truncated after reserves were consumed.
+        * As subsequent fault on such a range will not use reserves.
+        * Subtle - The reserve map for private mappings has the
+        * opposite meaning than that of shared mappings.  If NO
+        * entry is in the reserve map, it means a reservation exists.
+        * If an entry exists in the reserve map, it means the
+        * reservation has already been consumed.  As a result, the
+        * return value of this routine is the opposite of the
+        * value returned from reserve map manipulation routines above.
+        */
+       if (ret > 0)
+               return 0;
+       if (ret == 0)
+               return 1;
+       return ret;
 }