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SubjectRe: [External] Re: [PATCH v21 0/9] Free some vmemmap pages of HugeTLB page
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On 4/29/21 3:23 PM, Mike Kravetz wrote:
> On 4/28/21 9:02 PM, Muchun Song wrote:
>> On Thu, Apr 29, 2021 at 10:32 AM Mike Kravetz <mike.kravetz@oracle.com> wrote:
>>>
>>> On 4/28/21 5:26 AM, Muchun Song wrote:
>>>> On Wed, Apr 28, 2021 at 7:47 AM Mike Kravetz <mike.kravetz@oracle.com> wrote:
>>>>>
>>>>> Thanks! I will take a look at the modifications soon.
>>>>>
>>>>> I applied the patches to Andrew's mmotm-2021-04-21-23-03, ran some tests and
>>>>> got the following warning. We may need to special case that call to
>>>>> __prep_new_huge_page/free_huge_page_vmemmap from alloc_and_dissolve_huge_page
>>>>> as it is holding hugetlb lock with IRQs disabled.
>>>>
>>>> Good catch. Thanks Mike. I will fix it in the next version. How about this:
>>>>
>>>> @@ -1618,7 +1617,8 @@ static void __prep_new_huge_page(struct hstate
>>>> *h, struct page *page)
>>>>
>>>> static void prep_new_huge_page(struct hstate *h, struct page *page, int nid)
>>>> {
>>>> + free_huge_page_vmemmap(h, page);
>>>> __prep_new_huge_page(page);
>>>> spin_lock_irq(&hugetlb_lock);
>>>> __prep_account_new_huge_page(h, nid);
>>>> spin_unlock_irq(&hugetlb_lock);
>>>> @@ -2429,6 +2429,7 @@ static int alloc_and_dissolve_huge_page(struct
>>>> hstate *h, struct page *old_page,
>>>> if (!new_page)
>>>> return -ENOMEM;
>>>>
>>>> + free_huge_page_vmemmap(h, new_page);
>>>> retry:
>>>> spin_lock_irq(&hugetlb_lock);
>>>> if (!PageHuge(old_page)) {
>>>> @@ -2489,7 +2490,7 @@ static int alloc_and_dissolve_huge_page(struct
>>>> hstate *h, struct page *old_page,
>>>>
>>>> free_new:
>>>> spin_unlock_irq(&hugetlb_lock);
>>>> - __free_pages(new_page, huge_page_order(h));
>>>> + update_and_free_page(h, new_page, false);
>>>>
>>>> return ret;
>>>> }
>>>>
>>>>
>>>
>>> Another option would be to leave the prep* routines as is and only
>>> modify alloc_and_dissolve_huge_page as follows:
>>
>> OK. LGTM. I will use this. Thanks Mike.
>
> There are issues with my suggested patch below. I am occasionally
> hitting the BUG that checks for page ref count being zero at put_page
> time. Still do not fully understand, but I do not hit the same BUG
> with your patch above. Please do not use my patch below.
>

Ah! The issue is pretty obvious.


> @@ -2489,7 +2489,7 @@ static int alloc_and_dissolve_huge_page(struct hstate *h, struct page *old_page,
>
> free_new:
> spin_unlock_irq(&hugetlb_lock);
> - __free_pages(new_page, huge_page_order(h));
> + update_and_free_page(h, old_page, false);

That should of course be:
update_and_free_page(h, new_page, false);

>
> return ret;
> }
>

--
Mike Kravetz

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