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Message-ID: <a305816d-7770-47ec-bb21-1572c7c7783b@lucifer.local>
Date: Mon, 30 Jun 2025 13:57:50 +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 2/4] mm: Add batched versions of
ptep_modify_prot_start/commit
On Sat, Jun 28, 2025 at 05:04:33PM +0530, Dev Jain wrote:
> Batch ptep_modify_prot_start/commit in preparation for optimizing mprotect.
> Architecture can override these helpers; in case not, they are implemented
> as a simple loop over the corresponding single pte helpers.
>
> Signed-off-by: Dev Jain <dev.jain@....com>
Looks generally sensible! Some comments below.
> ---
> include/linux/pgtable.h | 83 ++++++++++++++++++++++++++++++++++++++++-
> mm/mprotect.c | 4 +-
> 2 files changed, 84 insertions(+), 3 deletions(-)
>
> diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
> index cf1515c163e2..662f39e7475a 100644
> --- a/include/linux/pgtable.h
> +++ b/include/linux/pgtable.h
> @@ -1331,7 +1331,8 @@ static inline pte_t ptep_modify_prot_start(struct vm_area_struct *vma,
>
> /*
> * Commit an update to a pte, leaving any hardware-controlled bits in
> - * the PTE unmodified.
> + * the PTE unmodified. The pte may have been "upgraded" w.r.t a/d bits compared
> + * to the old_pte, as in, it may have a/d bits on which were off in old_pte.
> */
> static inline void ptep_modify_prot_commit(struct vm_area_struct *vma,
> unsigned long addr,
> @@ -1340,6 +1341,86 @@ static inline void ptep_modify_prot_commit(struct vm_area_struct *vma,
> __ptep_modify_prot_commit(vma, addr, ptep, pte);
> }
> #endif /* __HAVE_ARCH_PTEP_MODIFY_PROT_TRANSACTION */
> +
> +/**
> + * modify_prot_start_ptes - Start a pte protection read-modify-write transaction
> + * over a batch of ptes, which protects against asynchronous hardware
> + * modifications to the ptes. The intention is not to prevent the hardware from
> + * making pte updates, but to prevent any updates it may make from being lost.
> + * Please see the comment above ptep_modify_prot_start() for full description.
> + *
> + * @vma: The virtual memory area the pages are mapped into.
> + * @addr: Address the first page is mapped at.
> + * @ptep: Page table pointer for the first entry.
> + * @nr: Number of entries.
> + *
> + * May be overridden by the architecture; otherwise, implemented as a simple
> + * loop over ptep_modify_prot_start(), collecting the a/d bits from each pte
> + * in the batch.
> + *
> + * Note that PTE bits in the PTE batch besides the PFN can differ.
> + *
> + * Context: The caller holds the page table lock. The PTEs map consecutive
> + * pages that belong to the same folio. The PTEs are all in the same PMD.
> + * Since the batch is determined from folio_pte_batch, the PTEs must differ
> + * only in a/d bits (and the soft dirty bit; see fpb_t flags in
> + * mprotect_folio_pte_batch()).
> + */
> +#ifndef modify_prot_start_ptes
> +static inline pte_t modify_prot_start_ptes(struct vm_area_struct *vma,
> + unsigned long addr, pte_t *ptep, unsigned int nr)
> +{
> + pte_t pte, tmp_pte;
> +
> + pte = ptep_modify_prot_start(vma, addr, ptep);
> + while (--nr) {
> + ptep++;
> + addr += PAGE_SIZE;
> + tmp_pte = ptep_modify_prot_start(vma, addr, ptep);
> + if (pte_dirty(tmp_pte))
> + pte = pte_mkdirty(pte);
> + if (pte_young(tmp_pte))
> + pte = pte_mkyoung(pte);
> + }
> + return pte;
> +}
> +#endif
> +
> +/**
> + * modify_prot_commit_ptes - Commit an update to a batch of ptes, leaving any
> + * hardware-controlled bits in the PTE unmodified.
> + *
> + * @vma: The virtual memory area the pages are mapped into.
> + * @addr: Address the first page is mapped at.
> + * @ptep: Page table pointer for the first entry.
> + * @old_pte: Old page table entry (for the first entry) which is now cleared.
> + * @pte: New page table entry to be set.
> + * @nr: Number of entries.
> + *
> + * May be overridden by the architecture; otherwise, implemented as a simple
> + * loop over ptep_modify_prot_commit().
> + *
> + * Context: The caller holds the page table lock. The PTEs are all in the same
> + * PMD. On exit, the set ptes in the batch map the same folio. The pte may have
> + * been "upgraded" w.r.t a/d bits compared to the old_pte, as in, it may have
> + * a/d bits on which were off in old_pte.
> + */
> +#ifndef modify_prot_commit_ptes
> +static inline void modify_prot_commit_ptes(struct vm_area_struct *vma, unsigned long addr,
> + pte_t *ptep, pte_t old_pte, pte_t pte, unsigned int nr)
> +{
> + int i;
> +
> + for (i = 0; i < nr; ++i) {
> + ptep_modify_prot_commit(vma, addr, ptep, old_pte, pte);
> + ptep++;
Weird place to put this increment, maybe just stick it in the for loop.
> + addr += PAGE_SIZE;
Same comment here.
> + old_pte = pte_next_pfn(old_pte);
Could be:
old_pte = pte;
No?
> + pte = pte_next_pfn(pte);
> + }
> +}
> +#endif
> +
> #endif /* CONFIG_MMU */
>
> /*
> diff --git a/mm/mprotect.c b/mm/mprotect.c
> index af10a7fbe6b8..627b0d67cc4a 100644
> --- a/mm/mprotect.c
> +++ b/mm/mprotect.c
> @@ -206,7 +206,7 @@ static long change_pte_range(struct mmu_gather *tlb,
> continue;
> }
>
> - oldpte = ptep_modify_prot_start(vma, addr, pte);
> + oldpte = modify_prot_start_ptes(vma, addr, pte, nr_ptes);
> ptent = pte_modify(oldpte, newprot);
>
> if (uffd_wp)
> @@ -232,7 +232,7 @@ static long change_pte_range(struct mmu_gather *tlb,
> can_change_pte_writable(vma, addr, ptent))
> ptent = pte_mkwrite(ptent, vma);
>
> - ptep_modify_prot_commit(vma, addr, pte, oldpte, ptent);
> + modify_prot_commit_ptes(vma, addr, pte, oldpte, ptent, nr_ptes);
> if (pte_needs_flush(oldpte, ptent))
> tlb_flush_pte_range(tlb, addr, PAGE_SIZE);
> pages++;
> --
> 2.30.2
>
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