linux-kernel - Re: [PATCH v1 1/2] mm: let pte_lockptr() consume a pte

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Message-ID: <db584d1e-a1b5-44f3-a1d3-61d2dbf88b63@redhat.com>
Date: Tue, 30 Jul 2024 18:10:37 +0200
From: David Hildenbrand <david@...hat.com>
To: Marek Szyprowski <m.szyprowski@...sung.com>, linux-kernel@...r.kernel.org
Cc: linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
 Muchun Song <muchun.song@...ux.dev>, Peter Xu <peterx@...hat.com>,
 Oscar Salvador <osalvador@...e.de>
Subject: Re: [PATCH v1 1/2] mm: let pte_lockptr() consume a pte_t pointer

On 30.07.24 18:08, Marek Szyprowski wrote:
> On 30.07.2024 17:49, David Hildenbrand wrote:
>> On 30.07.24 17:45, David Hildenbrand wrote:
>>> On 30.07.24 17:30, Marek Szyprowski wrote:
>>>> On 25.07.2024 20:39, David Hildenbrand wrote:
>>>>> pte_lockptr() is the only *_lockptr() function that doesn't consume
>>>>> what would be expected: it consumes a pmd_t pointer instead of a pte_t
>>>>> pointer.
>>>>>
>>>>> Let's change that. The two callers in pgtable-generic.c are easily
>>>>> adjusted. Adjust khugepaged.c:retract_page_tables() to simply do a
>>>>> pte_offset_map_nolock() to obtain the lock, even though we won't
>>>>> actually
>>>>> be traversing the page table.
>>>>>
>>>>> This makes the code more similar to the other variants and avoids
>>>>> other
>>>>> hacks to make the new pte_lockptr() version happy. pte_lockptr() users
>>>>> reside now only in  pgtable-generic.c.
>>>>>
>>>>> Maybe, using pte_offset_map_nolock() is the right thing to do because
>>>>> the PTE table could have been removed in the meantime? At least it
>>>>> sounds
>>>>> more future proof if we ever have other means of page table reclaim.
>>>>>
>>>>> It's not quite clear if holding the PTE table lock is really required:
>>>>> what if someone else obtains the lock just after we unlock it? But
>>>>> we'll
>>>>> leave that as is for now, maybe there are good reasons.
>>>>>
>>>>> This is a preparation for adapting hugetlb page table locking logic to
>>>>> take the same locks as core-mm page table walkers would.
>>>>>
>>>>> Signed-off-by: David Hildenbrand <david@...hat.com>
>>>>
>>>> This patch landed in today's linux-next as commit e98970a1d2d4 ("mm:
>>>> let
>>>> pte_lockptr() consume a pte_t pointer"). Unfortunately it causes the
>>>> following issue on most of my ARM 32bit based test boards:
>>>>
>>>
>>> That is ... rather surprising.
>>>
>>> The issue below seems to point at __pte_offset_map_lock(), where we
>>> essentially convert from
>>>
>>> ptlock_ptr(page_ptdesc(pmd_page(*pmd)));
>>>
>>> to
>>>
>>> ptlock_ptr(virt_to_ptdesc(pte));
>>
>> I'm wondering, is highmem involved here such that the PTE would be
>> kmap'ed and virt_to_page() would not do what we would expect it to do?
> 
> Yes, highmem is enabled on those boards and all of them have 1GB+ of
> RAM. For other kernel configuration options see
> arch/arm/configs/exynos_defconfig.

Yes, pretty sure that's it. virt_to_page() won't work on kmaped pages.
So looks like we cannot easily do the conversion in this patch. :(

We'll have to get hacky in patch #2 instead.

@Andrew, can you drop both patches for now? I'll send a v2 that
essentially does in v2 on top something like:
  

diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
index da800e56fe590..c2e330b1eee21 100644
--- a/include/linux/hugetlb.h
+++ b/include/linux/hugetlb.h
@@ -963,7 +963,13 @@ static inline spinlock_t *huge_pte_lockptr(struct hstate *h,
          * will give us the same result: the per-MM PT lock.
          */
         if (huge_page_size(h) < PMD_SIZE)
-               return pte_lockptr(mm, pte);
+               /*
+                * pte_lockptr() needs the PMD, which we don't have. Because of
+                * highmem we cannot convert pte_lockptr() to consume a pte.
+                * But as we never have highmem page tables in hugetlb, we can
+                * safely use virt_to_ptdesc() here.
+                */
+               return ptlock_ptr(virt_to_ptdesc(pte));
         else if (huge_page_size(h) < PUD_SIZE)
                 return pmd_lockptr(mm, (pmd_t *) pte);
         return pud_lockptr(mm, (pud_t *) pte);


-- 
Cheers,

David / dhildenb