linux-kernel - Re: [PATCH v4 1/4] mm: Optimize mprotect() for MM_CP_PROT

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Message-ID: <f7f12499-7b73-4209-a92e-91e04ffb0fdb@lucifer.local>
Date: Wed, 2 Jul 2025 10:37:53 +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 v4 1/4] mm: Optimize mprotect() for MM_CP_PROT_NUMA by
 batch-skipping PTEs

On Sat, Jun 28, 2025 at 05:04:32PM +0530, Dev Jain wrote:
> In case of prot_numa, there are various cases in which we can skip to the
> next iteration. Since the skip condition is based on the folio and not
> the PTEs, we can skip a PTE batch. Additionally refactor all of this
> into a new function to clean up the existing code.
>
> Signed-off-by: Dev Jain <dev.jain@....com>
> ---
>  mm/mprotect.c | 134 ++++++++++++++++++++++++++++++++------------------
>  1 file changed, 87 insertions(+), 47 deletions(-)
>
> diff --git a/mm/mprotect.c b/mm/mprotect.c
> index 88709c01177b..af10a7fbe6b8 100644
> --- a/mm/mprotect.c
> +++ b/mm/mprotect.c
> @@ -83,6 +83,83 @@ bool can_change_pte_writable(struct vm_area_struct *vma, unsigned long addr,
>  	return pte_dirty(pte);
>  }
>
> +static int mprotect_folio_pte_batch(struct folio *folio, unsigned long addr,
> +		pte_t *ptep, pte_t pte, int max_nr_ptes)
> +{
> +	const fpb_t flags = FPB_IGNORE_DIRTY | FPB_IGNORE_SOFT_DIRTY;
> +
> +	if (!folio || !folio_test_large(folio) || (max_nr_ptes == 1))
> +		return 1;
> +
> +	return folio_pte_batch(folio, addr, ptep, pte, max_nr_ptes, flags,
> +			       NULL, NULL, NULL);
> +}
> +
> +static int prot_numa_skip_ptes(struct folio **foliop, struct vm_area_struct *vma,
> +		unsigned long addr, pte_t oldpte, pte_t *pte, int target_node,
> +		int max_nr_ptes)
> +{

While it's nice to separate this out, it's not so nice to pass folio as a
pointer to a pointer like this and maybe or maybe not set it.

Just get the folio before you call this... you'll need it either way.

I'll wait until you separate it all out before reviewing next revision as a bit
tricky as-is.

Thanks!