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Message-ID: <1926967f-3805-2baf-6b86-24039c6513ca@huawei.com>
Date: Wed, 7 Apr 2021 11:09:25 +0800
From: Miaohe Lin <linmiaohe@...wei.com>
To: Mike Kravetz <mike.kravetz@...cle.com>, <akpm@...ux-foundation.org>
CC: <n-horiguchi@...jp.nec.com>, <hillf.zj@...baba-inc.com>,
<linux-kernel@...r.kernel.org>, <linux-mm@...ck.org>
Subject: Re: [PATCH 2/4] mm/hugeltb: simplify the return code of
__vma_reservation_common()
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@...wei.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;
}
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