linux-kernel - Re: [PATCH v5 6/7] mm: Optimize mprotect() by PTE batching

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Message-ID: <e7a52474-784f-4c7b-b4b5-d8dba30ddd75@redhat.com>
Date: Wed, 6 Aug 2025 12:12:46 +0200
From: David Hildenbrand <david@...hat.com>
To: Dev Jain <dev.jain@....com>, Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: akpm@...ux-foundation.org, ryan.roberts@....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 6/7] mm: Optimize mprotect() by PTE batching

On 06.08.25 12:04, Dev Jain wrote:
> 
> On 06/08/25 3:20 pm, Lorenzo Stoakes wrote:
>> On Wed, Aug 06, 2025 at 03:07:49PM +0530, Dev Jain wrote:
>>>>> You mean in _this_ PTE of the batch right? As we're invoking these
>>>>> on each part
>>>>> of the PTE table.
>>>>>
>>>>> I mean I guess we can simply do:
>>>>>
>>>>>      struct page *first_page = pte_page(ptent);
>>>>>
>>>>> Right?
>>>> Yes, but we should forward the result from vm_normal_page(), which does
>>>> exactly that for you, and increment the page accordingly as required,
>>>> just like with the pte we are processing.
>>> Makes sense, so I guess I will have to change the signature of
>>> prot_numa_skip()
>>>
>>> to pass a double ptr to a page instead of folio and derive the folio in the
>>> caller,
>>>
>>> and pass down both the folio and the page to
>>> set_write_prot_commit_flush_ptes.
>> I already don't love how we psas the folio back from there for very dubious
>> benefit. I really hate the idea of having a struct **page parameter...
>>
>> I wonder if we should just have a quick fixup for hotfix, and refine this more
>> later?
>>
>> I foresee some debate otherwise...
> 
> Yup I would personally prefer that. Although if you would like to see the churn, here
> it is (based on mm-hotfixes-unstable, untested):
> 
> ---
> mm/mprotect.c | 27 ++++++++++++++++-----------
> 1 file changed, 16 insertions(+), 11 deletions(-)
> diff --git a/mm/mprotect.c b/mm/mprotect.c
> index 78bded7acf79..0735870e89ab 100644
> --- a/mm/mprotect.c
> +++ b/mm/mprotect.c
> @@ -120,9 +120,10 @@ static int mprotect_folio_pte_batch(struct folio 
> *folio, pte_t *ptep,
> static bool prot_numa_skip(struct vm_area_struct *vma, unsigned long addr,
> pte_t oldpte, pte_t *pte, int target_node,
> - struct folio **foliop)
> + struct page **pagep)
> {
> struct folio *folio = NULL;
> + struct page *page = NULL;
> bool ret = true;
> bool toptier;
> int nid;
> @@ -131,7 +132,9 @@ static bool prot_numa_skip(struct vm_area_struct 
> *vma, unsigned long addr,
> if (pte_protnone(oldpte))
> goto skip;
> - folio = vm_normal_folio(vma, addr, oldpte);
> + page = vm_normal_page(vma, addr, oldpte);
> + if (page)
> + folio = page_folio(page);

See the draft I just send, where we move that vm_normal_page() into the 
caller.


-- 
Cheers,

David / dhildenb