| lists.openwall.net | lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC | |
|
Open Source and information security mailing list archives
| ||
|
Message-ID: <9687592f-ec04-410f-9fb2-9777edfe1178@arm.com>
Date: Wed, 30 Apr 2025 12:07:37 +0530
From: Dev Jain <dev.jain@....com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.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
On 29/04/25 6:49 pm, Lorenzo Stoakes wrote:
> 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()?
Because we won't be able to batch if the folio is NULL.
I think I really messed up by having separate patch 1 and 2. The real
intent of patch 1 was to do batching in patch 2 *and* not have insane
indentation. Perhaps I should merge them, or completely separate them
logically, I'll figure this out.
>
>> 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
>>
Powered by blists - more mailing lists