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 <[email protected]>
> 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 <[email protected]>
> Tested-by: Naoya Horiguchi <[email protected]>
> Reviewed-by: Naoya Horiguchi <[email protected]>
> Cc: <[email protected]>
> Signed-off-by: Andrew Morton <[email protected]>
> Signed-off-by: Linus Torvalds <[email protected]>
>
> 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
>
>
