linux-kernel - Re: [PATCH] mm/thp: Update mm_struct's MM

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Date:   Thu, 20 May 2021 19:17:04 -0700 (PDT)
From:   Hugh Dickins <hughd@...gle.com>
To:     Anshuman Khandual <anshuman.khandual@....com>
cc:     Andrew Morton <akpm@...ux-foundation.org>, Zi Yan <ziy@...dia.com>,
        Yang Shi <shy828301@...il.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH] mm/thp: Update mm_struct's MM_ANONPAGES stat for huge
 zero pages

On Tue, 18 May 2021, Anshuman Khandual wrote:

> Although the zero huge page is being shared across various processes, each
> mapping needs to update its mm_struct's MM_ANONPAGES stat by HPAGE_PMD_NR
> to be consistent. This just updates the stats in set_huge_zero_page() after
> the mapping gets created and in zap_huge_pmd() when mapping gets destroyed.
> 
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Zi Yan <ziy@...dia.com>
> Cc: linux-mm@...ck.org
> Cc: linux-kernel@...r.kernel.org
> Signed-off-by: Anshuman Khandual <anshuman.khandual@....com>

NAK.

For consistency with what? In the all the years that the huge zero page
has existed, it has been intentionally exempted from rss accounting:
consistent with the small zero page, which itself has always been
intentionally exempted from rss accounting. In fact, that's a good part
of the reason the huge zero page was introduced (see 4a6c1297268c).

To change that now will break any users depending on it.

Not to mention the
BUG: Bad rss-counter state mm:00000000aa61ef82 type:MM_ANONPAGES val:512
I just got from mmotm.

Hugh

> ---
> This applies on v5.13-rc2.
> 
> Changes in V1:
> 
> - Updated MM_ANONPAGES stat in zap_huge_pmd()
> - Updated the commit message
> 
> Changes in RFC:
> 
> https://lore.kernel.org/linux-mm/1620890438-9127-1-git-send-email-anshuman.khandual@arm.com/
> 
>  mm/huge_memory.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 63ed6b25deaa..306d0a41bf75 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -706,6 +706,7 @@ static void set_huge_zero_page(pgtable_t pgtable, struct mm_struct *mm,
>  	if (pgtable)
>  		pgtable_trans_huge_deposit(mm, pmd, pgtable);
>  	set_pmd_at(mm, haddr, pmd, entry);
> +	add_mm_counter(mm, MM_ANONPAGES, HPAGE_PMD_NR);
>  	mm_inc_nr_ptes(mm);
>  }
>  
> @@ -1678,6 +1679,7 @@ int zap_huge_pmd(struct mmu_gather *tlb, struct vm_area_struct *vma,
>  			tlb_remove_page_size(tlb, pmd_page(orig_pmd), HPAGE_PMD_SIZE);
>  	} else if (is_huge_zero_pmd(orig_pmd)) {
>  		zap_deposited_table(tlb->mm, pmd);
> +		add_mm_counter(tlb->mm, MM_ANONPAGES, -HPAGE_PMD_NR);
>  		spin_unlock(ptl);
>  		tlb_remove_page_size(tlb, pmd_page(orig_pmd), HPAGE_PMD_SIZE);
>  	} else {
> -- 
> 2.20.1