Re: Freeing HugeTLB page into buddy allocator

[Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>]

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.

Thanks,
Naoya Horiguchi

> 
> 
> commit a49ecbcd7b0d5a1cda7d60e03df402dd0ef76ac8
> Author: Jianguo Wu <wujianguo@huawei.com>
> Date:   Wed Dec 18 17:08:54 2013 -0800
> 
>     mm/memory-failure.c: recheck PageHuge() after hugetlb page migrate successfully
>     
>     After a successful hugetlb page migration by soft offline, the source
>     page will either be freed into hugepage_freelists or buddy(over-commit
>     page).  If page is in buddy, page_hstate(page) will be NULL.  It will
>     hit a NULL pointer dereference in dequeue_hwpoisoned_huge_page().
>     
>       BUG: unable to handle kernel NULL pointer dereference at 0000000000000058
>       IP: [<ffffffff81163761>] dequeue_hwpoisoned_huge_page+0x131/0x1d0
>       PGD c23762067 PUD c24be2067 PMD 0
>       Oops: 0000 [#1] SMP
>     
>     So check PageHuge(page) after call migrate_pages() successfully.
>     
>     Signed-off-by: Jianguo Wu <wujianguo@huawei.com>
>     Tested-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
>     Reviewed-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
>     Cc: <stable@vger.kernel.org>
>     Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
>     Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
> 
> diff --git a/mm/memory-failure.c b/mm/memory-failure.c
> index b7c1716..db08af9 100644
> --- a/mm/memory-failure.c
> +++ b/mm/memory-failure.c
> @@ -1505,10 +1505,16 @@ static int soft_offline_huge_page(struct page *page, int flags)
>  		if (ret > 0)
>  			ret = -EIO;
>  	} else {
> -		set_page_hwpoison_huge_page(hpage);
> -		dequeue_hwpoisoned_huge_page(hpage);
> -		atomic_long_add(1 << compound_order(hpage),
> -				&num_poisoned_pages);
> +		/* overcommit hugetlb page will be freed to buddy */
> +		if (PageHuge(page)) {
> +			set_page_hwpoison_huge_page(hpage);
> +			dequeue_hwpoisoned_huge_page(hpage);
> +			atomic_long_add(1 << compound_order(hpage),
> +					&num_poisoned_pages);
> +		} else {
> +			SetPageHWPoison(page);
> +			atomic_long_inc(&num_poisoned_pages);
> +		}
>  	}
>  	return ret;
>  }
> 
> Regards
> Anshuman
> 
> 
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