linux-kernel - Re: [PATCH 1/2] mm: hugetlb: Considering PMD sharing when flushing cache/TLBs

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Date:   Mon, 25 Apr 2022 17:16:08 -0700
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     Baolin Wang <baolin.wang@...ux.alibaba.com>,
        akpm@...ux-foundation.org
Cc:     almasrymina@...gle.com, songmuchun@...edance.com,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] mm: hugetlb: Considering PMD sharing when flushing
 cache/TLBs

On 4/24/22 07:50, Baolin Wang wrote:
> When moving hugetlb page tables, the cache flushing is called in
> move_page_tables() without considering the shared PMDs, which may
> be cause cache issues on some architectures.
> 
> Thus we should move the hugetlb cache flushing into
> move_hugetlb_page_tables() with considering the shared PMDs ranges,
> calculated by adjust_range_if_pmd_sharing_possible(). Meanwhile also
> expanding the TLBs flushing range in case of shared PMDs.
> 
> Note this is discovered via code inspection, and did not meet a real
> problem in practice so far.
> 
> Fixes: 550a7d60bd5e ("mm, hugepages: add mremap() support for hugepage backed vma")
> Signed-off-by: Baolin Wang <baolin.wang@...ux.alibaba.com>
> ---
>  mm/hugetlb.c | 17 +++++++++++++++--
>  mm/mremap.c  |  2 +-
>  2 files changed, 16 insertions(+), 3 deletions(-)

Thanks!

Reviewed-by: Mike Kravetz <mike.kravetz@...cle.com>
-- 
Mike Kravetz

> 
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 1945dfb..d3a6094 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -4937,10 +4937,17 @@ int move_hugetlb_page_tables(struct vm_area_struct *vma,
>  	unsigned long old_addr_copy;
>  	pte_t *src_pte, *dst_pte;
>  	struct mmu_notifier_range range;
> +	bool shared_pmd = false;
>  
>  	mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, vma, mm, old_addr,
>  				old_end);
>  	adjust_range_if_pmd_sharing_possible(vma, &range.start, &range.end);
> +	/*
> +	 * In case of shared PMDs, we should cover the maximum possible
> +	 * range.
> +	 */
> +	flush_cache_range(vma, range.start, range.end);
> +
>  	mmu_notifier_invalidate_range_start(&range);
>  	/* Prevent race with file truncation */
>  	i_mmap_lock_write(mapping);
> @@ -4957,8 +4964,10 @@ int move_hugetlb_page_tables(struct vm_area_struct *vma,
>  		 */
>  		old_addr_copy = old_addr;
>  
> -		if (huge_pmd_unshare(mm, vma, &old_addr_copy, src_pte))
> +		if (huge_pmd_unshare(mm, vma, &old_addr_copy, src_pte)) {
> +			shared_pmd = true;
>  			continue;
> +		}
>  
>  		dst_pte = huge_pte_alloc(mm, new_vma, new_addr, sz);
>  		if (!dst_pte)
> @@ -4966,7 +4975,11 @@ int move_hugetlb_page_tables(struct vm_area_struct *vma,
>  
>  		move_huge_pte(vma, old_addr, new_addr, src_pte, dst_pte);
>  	}
> -	flush_tlb_range(vma, old_end - len, old_end);
> +
> +	if (shared_pmd)
> +		flush_tlb_range(vma, range.start, range.end);
> +	else
> +		flush_tlb_range(vma, old_end - len, old_end);
>  	mmu_notifier_invalidate_range_end(&range);
>  	i_mmap_unlock_write(mapping);
>  
> diff --git a/mm/mremap.c b/mm/mremap.c
> index 98f50e6..0970025 100644
> --- a/mm/mremap.c
> +++ b/mm/mremap.c
> @@ -490,12 +490,12 @@ unsigned long move_page_tables(struct vm_area_struct *vma,
>  		return 0;
>  
>  	old_end = old_addr + len;
> -	flush_cache_range(vma, old_addr, old_end);
>  
>  	if (is_vm_hugetlb_page(vma))
>  		return move_hugetlb_page_tables(vma, new_vma, old_addr,
>  						new_addr, len);
>  
> +	flush_cache_range(vma, old_addr, old_end);
>  	mmu_notifier_range_init(&range, MMU_NOTIFY_UNMAP, 0, vma, vma->vm_mm,
>  				old_addr, old_end);
>  	mmu_notifier_invalidate_range_start(&range);