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

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Message-ID: <b74fcedb-60c5-4fd3-bcc7-74959e12c38d@redhat.com>
Date: Tue, 30 Jul 2024 23:07:43 +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 23:00, David Hildenbrand wrote:
> 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.

To be precise, the following on top:

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 1b1f40ff00b7d..f6c7fe8f5746f 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -2926,10 +2926,22 @@ 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)
+static inline struct page *ptep_pgtable_page(pte_t *pte)
  {
+       unsigned long mask = ~(PTRS_PER_PTE * sizeof(pte_t) - 1);
+
         BUILD_BUG_ON(IS_ENABLED(CONFIG_HIGHPTE));
-       return ptlock_ptr(virt_to_ptdesc(pte));
+       return virt_to_page((void *)((unsigned long)pte & mask));
+}
+
+static inline struct ptdesc *ptep_ptdesc(pte_t *pte)
+{
+       return page_ptdesc(ptep_pgtable_page(pte));
+}
+
+static inline spinlock_t *ptep_lockptr(struct mm_struct *mm, pte_t *pte)
+{
+       return ptlock_ptr(ptep_ptdesc(pte));
  }
  

virt_to_ptdesc() really is of limited use in core-mm code as it seems ...

-- 
Cheers,

David / dhildenb