Re: [PATCH v2 1/7] mm/thp: fix __split_huge_pmd_locked() for migration PMD

[Zi Yan]

On 2 Sep 2020, at 12:58, Ralph Campbell wrote:

> A migrating transparent huge page has to already be unmapped. Otherwise,
> the page could be modified while it is being copied to a new page and
> data could be lost. The function __split_huge_pmd() checks for a PMD
> migration entry before calling __split_huge_pmd_locked() leading one to
> think that __split_huge_pmd_locked() can handle splitting a migrating PMD.
> However, the code always increments the page->_mapcount and adjusts the
> memory control group accounting assuming the page is mapped.
> Also, if the PMD entry is a migration PMD entry, the call to
> is_huge_zero_pmd(*pmd) is incorrect because it calls pmd_pfn(pmd) instead
> of migration_entry_to_pfn(pmd_to_swp_entry(pmd)).
> Fix these problems by checking for a PMD migration entry.
>
> Signed-off-by: Ralph Campbell <rcampbell@xxxxxxxxxx>

Thanks for the fix. You can add Reviewed-by: Zi Yan <ziy@xxxxxxxxxx>

I think you also want to add the Fixes tag and cc stable.

Fixes 84c3fc4e9c56 (“mm: thp: check pmd migration entry in common path”)
cc: stable@xxxxxxxxxxxxxxx # 4.14+

> ---
>  mm/huge_memory.c | 42 +++++++++++++++++++++++-------------------
>  1 file changed, 23 insertions(+), 19 deletions(-)
>
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 2a468a4acb0a..606d712d9505 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -2023,7 +2023,7 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
>  		put_page(page);
>  		add_mm_counter(mm, mm_counter_file(page), -HPAGE_PMD_NR);
>  		return;
> -	} else if (is_huge_zero_pmd(*pmd)) {
> +	} else if (pmd_trans_huge(*pmd) && is_huge_zero_pmd(*pmd)) {
>  		/*
>  		 * FIXME: Do we want to invalidate secondary mmu by calling
>  		 * mmu_notifier_invalidate_range() see comments below inside
> @@ -2117,30 +2117,34 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
>  		pte = pte_offset_map(&_pmd, addr);
>  		BUG_ON(!pte_none(*pte));
>  		set_pte_at(mm, addr, pte, entry);
> -		atomic_inc(&page[i]._mapcount);
> -		pte_unmap(pte);
> -	}
> -
> -	/*
> -	 * Set PG_double_map before dropping compound_mapcount to avoid
> -	 * false-negative page_mapped().
> -	 */
> -	if (compound_mapcount(page) > 1 && !TestSetPageDoubleMap(page)) {
> -		for (i = 0; i < HPAGE_PMD_NR; i++)
> +		if (!pmd_migration)
>  			atomic_inc(&page[i]._mapcount);
> +		pte_unmap(pte);
>  	}
>
> -	lock_page_memcg(page);
> -	if (atomic_add_negative(-1, compound_mapcount_ptr(page))) {
> -		/* Last compound_mapcount is gone. */
> -		__dec_lruvec_page_state(page, NR_ANON_THPS);
> -		if (TestClearPageDoubleMap(page)) {
> -			/* No need in mapcount reference anymore */
> +	if (!pmd_migration) {
> +		/*
> +		 * Set PG_double_map before dropping compound_mapcount to avoid
> +		 * false-negative page_mapped().
> +		 */
> +		if (compound_mapcount(page) > 1 &&
> +		    !TestSetPageDoubleMap(page)) {
>  			for (i = 0; i < HPAGE_PMD_NR; i++)
> -				atomic_dec(&page[i]._mapcount);
> +				atomic_inc(&page[i]._mapcount);
> +		}
> +
> +		lock_page_memcg(page);
> +		if (atomic_add_negative(-1, compound_mapcount_ptr(page))) {
> +			/* Last compound_mapcount is gone. */
> +			__dec_lruvec_page_state(page, NR_ANON_THPS);
> +			if (TestClearPageDoubleMap(page)) {
> +				/* No need in mapcount reference anymore */
> +				for (i = 0; i < HPAGE_PMD_NR; i++)
> +					atomic_dec(&page[i]._mapcount);
> +			}
>  		}
> +		unlock_page_memcg(page);
>  	}
> -	unlock_page_memcg(page);
>
>  	smp_wmb(); /* make pte visible before pmd */
>  	pmd_populate(mm, pmd, pgtable);
> -- 
> 2.20.1


—
Best Regards,
Yan Zi

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