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Message-ID: <f4a90024-3cc7-4536-84b0-665021d96125@lucifer.local>
Date: Tue, 29 Apr 2025 14:19:47 +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, namit@...are.com,
hughd@...gle.com, yang@...amperecomputing.com, ziy@...dia.com
Subject: Re: [PATCH v2 2/7] mm: Optimize mprotect() by batch-skipping PTEs
Very very very nitty on subject (sorry I realise this is annoying :P) -
generally don't need to capitalise 'Optimize' here :>)
Generally I like the idea here. But some issues on impl.
On Tue, Apr 29, 2025 at 10:53:31AM +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.
>
> Signed-off-by: Dev Jain <dev.jain@....com>
> ---
> mm/mprotect.c | 27 ++++++++++++++++++++-------
> 1 file changed, 20 insertions(+), 7 deletions(-)
>
> diff --git a/mm/mprotect.c b/mm/mprotect.c
> index 70f59aa8c2a8..ec5d17af7650 100644
> --- a/mm/mprotect.c
> +++ b/mm/mprotect.c
> @@ -91,6 +91,9 @@ static bool prot_numa_skip(struct vm_area_struct *vma, struct folio *folio,
> bool toptier;
> int nid;
>
> + if (folio_is_zone_device(folio) || folio_test_ksm(folio))
> + return true;
> +
Hm why not just put this here from the start? I think you should put this back
in the prior commit.
> /* Also skip shared copy-on-write pages */
> if (is_cow_mapping(vma->vm_flags) &&
> (folio_maybe_dma_pinned(folio) ||
> @@ -126,8 +129,10 @@ static bool prot_numa_skip(struct vm_area_struct *vma, struct folio *folio,
> }
>
> static bool prot_numa_avoid_fault(struct vm_area_struct *vma,
> - unsigned long addr, pte_t oldpte, int target_node)
> + unsigned long addr, pte_t *pte, pte_t oldpte, int target_node,
> + int max_nr, int *nr)
Hate this ptr to nr.
Why not just return nr, if it's 0 then skip? Simple!
> {
> + const fpb_t flags = FPB_IGNORE_DIRTY | FPB_IGNORE_SOFT_DIRTY;
> struct folio *folio;
> int ret;
>
> @@ -136,12 +141,16 @@ static bool prot_numa_avoid_fault(struct vm_area_struct *vma,
> return true;
>
> folio = vm_normal_folio(vma, addr, oldpte);
> - if (!folio || folio_is_zone_device(folio) ||
> - folio_test_ksm(folio))
> + if (!folio)
> return true;
> +
Very nitty, but stray extra line unless intended...
Not sure why we can't just put this !folio check in prot_numa_skip()?
> ret = prot_numa_skip(vma, folio, target_node);
> - if (ret)
> + if (ret) {
> + if (folio_test_large(folio) && max_nr != 1)
> + *nr = folio_pte_batch(folio, addr, pte, oldpte,
> + max_nr, flags, NULL, NULL, NULL);
So max_nr can <= 0 too? Shouldn't this be max_nr > 1?
> return ret;
Again x = fn_return_bool(); if (x) { return x; } is a bit silly, just do if
(fn_return_bool()) { return true; }.
If we return the number of pages, then this can become really simple, like:
I feel like maybe we should abstract the folio large handling here, though it'd
be a tiny function so hm.
Anyway assuming we leave it in place, and return number of pages processed, this
can become:
if (prot_numa_skip(vma, folio, target_node)) {
if (folio_test_large(folio) && max_nr > 1)
return folio_pte_batch(folio, addr, pte, oldpte, max_nr, flags,
NULL, NULL, NULL);
return 1;
}
Which is neater I think!
> + }
> if (folio_use_access_time(folio))
> folio_xchg_access_time(folio,
> jiffies_to_msecs(jiffies));
> @@ -159,6 +168,7 @@ static long change_pte_range(struct mmu_gather *tlb,
> bool prot_numa = cp_flags & MM_CP_PROT_NUMA;
> bool uffd_wp = cp_flags & MM_CP_UFFD_WP;
> bool uffd_wp_resolve = cp_flags & MM_CP_UFFD_WP_RESOLVE;
> + int nr;
>
> tlb_change_page_size(tlb, PAGE_SIZE);
> pte = pte_offset_map_lock(vma->vm_mm, pmd, addr, &ptl);
> @@ -173,8 +183,10 @@ static long change_pte_range(struct mmu_gather *tlb,
> flush_tlb_batched_pending(vma->vm_mm);
> arch_enter_lazy_mmu_mode();
> do {
> + nr = 1;
> oldpte = ptep_get(pte);
> if (pte_present(oldpte)) {
> + int max_nr = (end - addr) >> PAGE_SHIFT;
Not a fan of open-coding this. Since we already provide addr, why not just
provide end as well and have prot_numa_avoid_fault() calculate it?
> pte_t ptent;
>
> /*
> @@ -182,8 +194,9 @@ static long change_pte_range(struct mmu_gather *tlb,
> * pages. See similar comment in change_huge_pmd.
> */
> if (prot_numa &&
> - prot_numa_avoid_fault(vma, addr,
> - oldpte, target_node))
> + prot_numa_avoid_fault(vma, addr, pte,
> + oldpte, target_node,
> + max_nr, &nr))
> continue;
>
> oldpte = ptep_modify_prot_start(vma, addr, pte);
> @@ -300,7 +313,7 @@ static long change_pte_range(struct mmu_gather *tlb,
> pages++;
> }
> }
> - } while (pte++, addr += PAGE_SIZE, addr != end);
> + } while (pte += nr, addr += nr * PAGE_SIZE, addr != end);
This is icky, having 'nr' here like this.
But alternatives might be _even more_ icky (that is advancing both on
prot_numa_avoid_fault() so probably we need to keep it like this.
Maybe more a moan at the C programming language tbh haha!
> arch_leave_lazy_mmu_mode();
> pte_unmap_unlock(pte - 1, ptl);
>
> --
> 2.30.2
>
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