linux-kernel - Re: [PATCH v3 5/6] mm/hugetlb: add migration entry check in __unmap_hugepage

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Date:	Wed, 3 Sep 2014 18:47:38 -0700 (PDT)
From:	Hugh Dickins <hughd@...gle.com>
To:	Naoya Horiguchi <n-horiguchi@...jp.nec.com>
cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Hugh Dickins <hughd@...gle.com>,
	David Rientjes <rientjes@...gle.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	Naoya Horiguchi <nao.horiguchi@...il.com>
Subject: Re: [PATCH v3 5/6] mm/hugetlb: add migration entry check in
 __unmap_hugepage_range

On Thu, 28 Aug 2014, Naoya Horiguchi wrote:

> If __unmap_hugepage_range() tries to unmap the address range over which
> hugepage migration is on the way, we get the wrong page because pte_page()
> doesn't work for migration entries. This patch calls pte_to_swp_entry() and
> migration_entry_to_page() to get the right page for migration entries.
> 
> Signed-off-by: Naoya Horiguchi <n-horiguchi@...jp.nec.com>
> Cc: <stable@...r.kernel.org>  # [2.6.36+]

2.6.36+?  But this one doesn't affect hwpoisoned.
I admit I've lost track of how far back hugetlb migration goes:
oh, to 2.6.37+, that fits with what you marked on some commits earlier.
But then 2/6 says 3.12+.  Help!  Please remind me of the sequence of events.

> ---
>  mm/hugetlb.c | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git mmotm-2014-08-25-16-52.orig/mm/hugetlb.c mmotm-2014-08-25-16-52/mm/hugetlb.c
> index 1ed9df6def54..0a4511115ee0 100644
> --- mmotm-2014-08-25-16-52.orig/mm/hugetlb.c
> +++ mmotm-2014-08-25-16-52/mm/hugetlb.c
> @@ -2652,6 +2652,13 @@ void __unmap_hugepage_range(struct mmu_gather *tlb, struct vm_area_struct *vma,
>  		if (huge_pte_none(pte))
>  			goto unlock;
>  
> +		if (unlikely(is_hugetlb_entry_migration(pte))) {
> +			swp_entry_t entry = pte_to_swp_entry(pte);
> +
> +			page = migration_entry_to_page(entry);
> +			goto clear;
> +		}
> +

This surprises me: are you sure?  Obviously you know hugetlb migration
much better than I do: is it done in a significantly different way from
order:0 page migration?  In the order:0 case, there is no reference to
the page corresponding to the migration entry placed in a page table,
just the remaining reference held by the task doing the migration.  But
here you are jumping to the code which unmaps and frees a present page.

I can see that a fix is necessary, but I would have expected it to
consist of merely changing the "HWPoisoned" comment below to include
migration entries, and changing its test from
		if (unlikely(is_hugetlb_entry_hwpoisoned(pte))) {
to
		if (unlikely(!pte_present(pte))) {

>  		/*
>  		 * HWPoisoned hugepage is already unmapped and dropped reference
>  		 */
> @@ -2677,7 +2684,7 @@ void __unmap_hugepage_range(struct mmu_gather *tlb, struct vm_area_struct *vma,
>  			 */
>  			set_vma_resv_flags(vma, HPAGE_RESV_UNMAPPED);
>  		}
> -
> +clear:
>  		pte = huge_ptep_get_and_clear(mm, address, ptep);
>  		tlb_remove_tlb_entry(tlb, ptep, address);
>  		if (huge_pte_dirty(pte))
> -- 
> 1.9.3
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