linux-kernel - Re: Freeing HugeTLB page into buddy allocator

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Date:   Thu, 27 Apr 2017 16:36:20 +0530
From:   Anshuman Khandual <khandual@...ux.vnet.ibm.com>
To:     Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
        Anshuman Khandual <khandual@...ux.vnet.ibm.com>
Cc:     "wujianguo@...wei.com" <wujianguo@...wei.com>,
        Linux Memory Management List <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Freeing HugeTLB page into buddy allocator

On 04/27/2017 11:24 AM, Naoya Horiguchi wrote:
> On Tue, Apr 25, 2017 at 02:27:27PM +0530, Anshuman Khandual wrote:
>> Hello Jianguo,
>>
>> In the commit a49ecbcd7b0d5a1cda, it talks about HugeTLB page being
>> freed into buddy allocator instead of hugepage_freelists. But if
>> I look the code closely for the function unmap_and_move_huge_page()
>> it only calls putback_active_hugepage() which puts the page into the
>> huge page active list to free up the source HugeTLB page after any
>> successful migration. I might be missing something here, so can you
>> please point me where we release the HugeTLB page into buddy allocator
>> directly during migration ?
> 
> Hi Anshuman,
> 
> As stated in the patch description, source hugetlb page is freed after
> successful migration if overcommit is configured.
> 
> The call chain is like below:
> 
>   soft_offline_huge_page
>     migrate_pages
>       unmap_and_move_huge_page
>         putback_active_hugepage(hpage)
>           put_page // refcount is down to 0
>             __put_page
>               __put_compound_page
>                 free_huge_page
>                   if (h->surplus_huge_pages_node[nid])
>                     update_and_free_page
>                       __free_pages
> 
> So the inline comment
> 
> +		/* overcommit hugetlb page will be freed to buddy */
> 
> might be confusing because at this point the overcommit hugetlb page was
> already freed to buddy.
> 
> I hope this will help you.

Surely does. Thanks Naoya.