linux-kernel - Re: [PATCH v2] mm/hugetlb: fix hugetlb vs. core-mm PT locking

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Message-ID: <3f6c97b5-ccd8-4226-a9ac-78d555b0d048@redhat.com>
Date: Tue, 30 Jul 2024 23:00:00 +0200
From: David Hildenbrand <david@...hat.com>
To: James Houghton <jthoughton@...gle.com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org, stable@...r.kernel.org,
 Peter Xu <peterx@...hat.com>, Oscar Salvador <osalvador@...e.de>,
 Muchun Song <muchun.song@...ux.dev>,
 Baolin Wang <baolin.wang@...ux.alibaba.com>
Subject: Re: [PATCH v2] mm/hugetlb: fix hugetlb vs. core-mm PT locking

On 30.07.24 22:43, James Houghton wrote:
> On Tue, Jul 30, 2024 at 1:03 PM David Hildenbrand <david@...hat.com> wrote:
>> diff --git a/include/linux/mm.h b/include/linux/mm.h
>> index b100df8cb5857..1b1f40ff00b7d 100644
>> --- a/include/linux/mm.h
>> +++ b/include/linux/mm.h
>> @@ -2926,6 +2926,12 @@ static inline spinlock_t *pte_lockptr(struct mm_struct *mm, pmd_t *pmd)
>>          return ptlock_ptr(page_ptdesc(pmd_page(*pmd)));
>>   }
>>
>> +static inline spinlock_t *ptep_lockptr(struct mm_struct *mm, pte_t *pte)
>> +{
>> +       BUILD_BUG_ON(IS_ENABLED(CONFIG_HIGHPTE));
>> +       return ptlock_ptr(virt_to_ptdesc(pte));
> 
> Hi David,
> 

Hi!

> Small question: ptep_lockptr() does not handle the case where the size
> of the PTE table is larger than PAGE_SIZE, but pmd_lockptr() does.

I thought I convinced myself that leaf page tables are always single 
pages and had a comment in v1.

But now I have to double-check again, and staring at 
pagetable_pte_ctor() callers I am left confused.

It certainly sounds more future proof to just align the pointer down to 
the start of the PTE table like pmd_lockptr() would.

> IIUC, for pte_lockptr() and ptep_lockptr() to return the same result
> in this case, ptep_lockptr() should be doing the masking that
> pmd_lockptr() is doing. Are you sure that you don't need to be doing
> it? (Or maybe I am misunderstanding something.)

It's a valid concern even if it would not be required. But I'm afraid I 
won't dig into the details and simply do the alignment in a v3.

I'm hoping I'll be done with that hugetlb crap soon; it's starting to 
annoy me and I really should be working on other stuff ...

-- 
Cheers,

David / dhildenb