linux-kernel - Re: [PATCH v3 4/7] mm/thp: Carry over dirty bit when thp splits on pmd

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Date:   Wed, 10 Aug 2022 14:24:33 +0800
From:   "Huang, Ying" <ying.huang@...el.com>
To:     Peter Xu <peterx@...hat.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Minchan Kim <minchan@...nel.org>,
        David Hildenbrand <david@...hat.com>,
        Nadav Amit <nadav.amit@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Hugh Dickins <hughd@...gle.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andi Kleen <andi.kleen@...el.com>,
        "Kirill A . Shutemov" <kirill@...temov.name>
Subject: Re: [PATCH v3 4/7] mm/thp: Carry over dirty bit when thp splits on pmd

Peter Xu <peterx@...hat.com> writes:

> Carry over the dirty bit from pmd to pte when a huge pmd splits.  It
> shouldn't be a correctness issue since when pmd_dirty() we'll have the page
> marked dirty anyway, however having dirty bit carried over helps the next
> initial writes of split ptes on some archs like x86.
>
> Signed-off-by: Peter Xu <peterx@...hat.com>
> ---
>  mm/huge_memory.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
>
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 0611b2fd145a..e8e78d1bac5f 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -2005,7 +2005,7 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
>  	pgtable_t pgtable;
>  	pmd_t old_pmd, _pmd;
>  	bool young, write, soft_dirty, pmd_migration = false, uffd_wp = false;
> -	bool anon_exclusive = false;
> +	bool anon_exclusive = false, dirty = false;
>  	unsigned long addr;
>  	int i;
>  
> @@ -2098,6 +2098,7 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
>  			SetPageDirty(page);
>  		write = pmd_write(old_pmd);
>  		young = pmd_young(old_pmd);
> +		dirty = pmd_dirty(old_pmd);

Nitpick: This can be put under

		if (pmd_dirty(old_pmd))
			SetPageDirty(page);

Not a big deal.

Reviewed-by: "Huang, Ying" <ying.huang@...el.com>

>  		soft_dirty = pmd_soft_dirty(old_pmd);
>  		uffd_wp = pmd_uffd_wp(old_pmd);
>  
> @@ -2161,6 +2162,9 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
>  				entry = pte_wrprotect(entry);
>  			if (!young)
>  				entry = pte_mkold(entry);
> +			/* NOTE: this may set soft-dirty too on some archs */
> +			if (dirty)
> +				entry = pte_mkdirty(entry);
>  			if (soft_dirty)
>  				entry = pte_mksoft_dirty(entry);
>  			if (uffd_wp)