linux-kernel - Re: [PATCH v4 12/13] mm: multi-gen LRU: walk_pte_range() use pte_offset_map_rw

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Message-ID: <090c6ebc-c5cd-4d72-989d-40ee03ec1297@redhat.com>
Date: Tue, 24 Sep 2024 16:39:15 +0200
From: David Hildenbrand <david@...hat.com>
To: Qi Zheng <zhengqi.arch@...edance.com>, hughd@...gle.com,
 willy@...radead.org, muchun.song@...ux.dev, vbabka@...nel.org,
 akpm@...ux-foundation.org, rppt@...nel.org, vishal.moola@...il.com,
 peterx@...hat.com, ryan.roberts@....com, christophe.leroy2@...soprasteria.com
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org,
 linux-arm-kernel@...ts.infradead.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH v4 12/13] mm: multi-gen LRU: walk_pte_range() use
 pte_offset_map_rw_nolock()

On 24.09.24 08:10, Qi Zheng wrote:
> In walk_pte_range(), we may modify the pte entry after holding the ptl, so
> convert it to using pte_offset_map_rw_nolock(). At this time, the
> pte_same() check is not performed after the ptl held, so we should get
> pmdval and do pmd_same() check to ensure the stability of pmd entry.
> 
> Signed-off-by: Qi Zheng <zhengqi.arch@...edance.com>
> Reviewed-by: Muchun Song <muchun.song@...ux.dev>
> ---
>   mm/vmscan.c | 9 ++++++++-
>   1 file changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 749cdc110c745..bdca94e663bc5 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -3375,8 +3375,10 @@ static bool walk_pte_range(pmd_t *pmd, unsigned long start, unsigned long end,
>   	struct pglist_data *pgdat = lruvec_pgdat(walk->lruvec);
>   	DEFINE_MAX_SEQ(walk->lruvec);
>   	int old_gen, new_gen = lru_gen_from_seq(max_seq);
> +	pmd_t pmdval;
>   
> -	pte = pte_offset_map_nolock(args->mm, pmd, start & PMD_MASK, &ptl);
> +	pte = pte_offset_map_rw_nolock(args->mm, pmd, start & PMD_MASK, &pmdval,
> +				       &ptl);
>   	if (!pte)
>   		return false;
>   	if (!spin_trylock(ptl)) {
> @@ -3384,6 +3386,11 @@ static bool walk_pte_range(pmd_t *pmd, unsigned long start, unsigned long end,
>   		return false;
>   	}
>   
> +	if (unlikely(!pmd_same(pmdval, pmdp_get_lockless(pmd)))) {
> +		pte_unmap_unlock(pte, ptl);
> +		return false;
> +	}

Acked-by: David Hildenbrand <david@...hat.com>

-- 
Cheers,

David / dhildenb