linux-kernel - Re: [PATCH v5 4/7] mm: Introduce FPB_RESPECT

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Message-ID: <149e5957-80ec-4b73-936f-6ab7e67cb6ca@lucifer.local>
Date: Fri, 18 Jul 2025 18:12:11 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Dev Jain <dev.jain@....com>
Cc: akpm@...ux-foundation.org, ryan.roberts@....com, david@...hat.com,
        willy@...radead.org, linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        catalin.marinas@....com, will@...nel.org, Liam.Howlett@...cle.com,
        vbabka@...e.cz, jannh@...gle.com, anshuman.khandual@....com,
        peterx@...hat.com, joey.gouly@....com, ioworker0@...il.com,
        baohua@...nel.org, kevin.brodsky@....com, quic_zhenhuah@...cinc.com,
        christophe.leroy@...roup.eu, yangyicong@...ilicon.com,
        linux-arm-kernel@...ts.infradead.org, hughd@...gle.com,
        yang@...amperecomputing.com, ziy@...dia.com
Subject: Re: [PATCH v5 4/7] mm: Introduce FPB_RESPECT_WRITE for PTE batching
 infrastructure

On Fri, Jul 18, 2025 at 02:32:41PM +0530, Dev Jain wrote:
> Patch 6 optimizes mprotect() by batch clearing the ptes, masking in the new
> protections, and batch setting the ptes. Suppose that the first pte
> of the batch is writable - with the current implementation of
> folio_pte_batch(), it is not guaranteed that the other ptes in the batch
> are already writable too, so we may incorrectly end up setting the
> writable bit on all ptes via modify_prot_commit_ptes().
>
> Therefore, introduce FPB_RESPECT_WRITE so that all ptes in the batch
> are writable or not.
>
> Signed-off-by: Dev Jain <dev.jain@....com>

LGTM, so:

Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>

> ---
>  mm/internal.h | 11 ++++++++---
>  1 file changed, 8 insertions(+), 3 deletions(-)
>
> diff --git a/mm/internal.h b/mm/internal.h
> index 5b0f71e5434b..28d2d5b051df 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -208,17 +208,20 @@ typedef int __bitwise fpb_t;
>  /* Compare PTEs respecting the soft-dirty bit. */
>  #define FPB_RESPECT_SOFT_DIRTY		((__force fpb_t)BIT(1))
>
> +/* Compare PTEs respecting the writable bit. */
> +#define FPB_RESPECT_WRITE		((__force fpb_t)BIT(2))
> +
>  /*
>   * Merge PTE write bits: if any PTE in the batch is writable, modify the
>   * PTE at @ptentp to be writable.
>   */
> -#define FPB_MERGE_WRITE			((__force fpb_t)BIT(2))
> +#define FPB_MERGE_WRITE			((__force fpb_t)BIT(3))
>
>  /*
>   * Merge PTE young and dirty bits: if any PTE in the batch is young or dirty,
>   * modify the PTE at @ptentp to be young or dirty, respectively.
>   */
> -#define FPB_MERGE_YOUNG_DIRTY		((__force fpb_t)BIT(3))
> +#define FPB_MERGE_YOUNG_DIRTY		((__force fpb_t)BIT(4))
>
>  static inline pte_t __pte_batch_clear_ignored(pte_t pte, fpb_t flags)
>  {
> @@ -226,7 +229,9 @@ static inline pte_t __pte_batch_clear_ignored(pte_t pte, fpb_t flags)
>  		pte = pte_mkclean(pte);
>  	if (likely(!(flags & FPB_RESPECT_SOFT_DIRTY)))
>  		pte = pte_clear_soft_dirty(pte);
> -	return pte_wrprotect(pte_mkold(pte));
> +	if (likely(!(flags & FPB_RESPECT_WRITE)))
> +		pte = pte_wrprotect(pte);
> +	return pte_mkold(pte);
>  }
>
>  /**
> --
> 2.30.2
>