Re: [syzbot ci] Re: Fix stale IOTLB entries for kernel address space

[David Hildenbrand <david@redhat.com>]

On 20.10.25 07:34, Baolu Lu wrote:
> On 10/18/25 01:10, David Hildenbrand wrote:
>> On 16.10.25 10:00, Baolu Lu wrote:
>>> On 10/16/25 00:25, Dave Hansen wrote:
>>>> Here's the part that confuses me:
>>>>
>>>> On 10/14/25 13:59, syzbot ci wrote:
>>>>> page last free pid 5965 tgid 5964 stack trace:
>>>>>     reset_page_owner include/linux/page_owner.h:25 [inline]
>>>>>     free_pages_prepare mm/page_alloc.c:1394 [inline]
>>>>>     __free_frozen_pages+0xbc4/0xd30 mm/page_alloc.c:2906
>>>>>     pmd_free_pte_page+0xa1/0xc0 arch/x86/mm/pgtable.c:783
>>>>>     vmap_try_huge_pmd mm/vmalloc.c:158 [inline]
>>>> ...
>>>>
>>>> So, vmap_try_huge_pmd() did a pmd_free_pte_page(). Yet, somehow, the PMD
>>>> stuck around so that it *could* be used after being freed. It _looks_
>>>> like pmd_free_pte_page() freed the page, returned 0, and made
>>>> vmap_try_huge_pmd() return early, skipping the pmd pmd_set_huge().
>>>>
>>>> But I don't know how that could possibly happen.
>>>
>>> The reported issue is only related to this patch:
>>>
>>> - [PATCH v6 3/7] x86/mm: Use 'ptdesc' when freeing PMD pages
>>>
>>> It appears that the pmd_ptdesc() helper can't be used directly here in
>>> this patch. pmd_ptdesc() retrieves the page table page that the PMD
>>> entry resides in:
>>>
>>> static inline struct page *pmd_pgtable_page(pmd_t *pmd)
>>> {
>>>            unsigned long mask = ~(PTRS_PER_PMD * sizeof(pmd_t) - 1);
>>>            return virt_to_page((void *)((unsigned long) pmd & mask));
>>> }
>>>
>>> static inline struct ptdesc *pmd_ptdesc(pmd_t *pmd)
>>> {
>>>            return page_ptdesc(pmd_pgtable_page(pmd));
>>> }
>>>
>>> while, in this patch, we need the page descriptor that a pmd entry
>>> points to.
>>
>> Ah. But that's just pointing at a leaf page table, right?
> 
> Yes, that points to a leaf page table.

Right, so I guess there is no simplifying that.

Like we have in pte_lockptr():

	return ptlock_ptr(page_ptdesc(pmd_page(*pmd)));

So page_ptdesc(pmd_page()) is the way to go for now.

Sorry for the confusion.

-- 
Cheers

David / dhildenb