Re: [PATCH -next 1/1] mm: hugetlb_vmemmap: Fix WARN_ON in vmemmap_remap_pte

[Muchun Song]

> On Oct 26, 2022, at 16:36, Anshuman Khandual <anshuman.khandual@xxxxxxx> wrote:
> 
> 
> 
> On 10/26/22 12:31, Muchun Song wrote:
>> 
>> 
>>> On Oct 26, 2022, at 13:06, Anshuman Khandual <anshuman.khandual@xxxxxxx> wrote:
>>> 
>>> 
>>> 
>>> On 10/25/22 12:06, Muchun Song wrote:
>>>> 
>>>> 
>>>>> On Oct 25, 2022, at 09:42, Wupeng Ma <mawupeng1@xxxxxxxxxx> wrote:
>>>>> 
>>>>> From: Ma Wupeng <mawupeng1@xxxxxxxxxx>
>>>>> 
>>>>> Commit f41f2ed43ca5 ("mm: hugetlb: free the vmemmap pages associated with
>>>>> each HugeTLB page") add vmemmap_remap_pte to remap the tail pages as
>>>>> read-only to catch illegal write operation to the tail page.
>>>>> 
>>>>> However this will lead to WARN_ON in arm64 in __check_racy_pte_update()
>>>> 
>>>> Thanks for your finding this issue.
>>>> 
>>>>> since this may lead to dirty state cleaned. This check is introduced by
>>>>> commit 2f4b829c625e ("arm64: Add support for hardware updates of the
>>>>> access and dirty pte bits") and the initial check is as follow:
>>>>> 
>>>>> BUG_ON(pte_write(*ptep) && !pte_dirty(pte));
>>>>> 
>>>>> Since we do need to mark this pte as read-only to catch illegal write
>>>>> operation to the tail pages, use set_pte  to replace set_pte_at to bypass
>>>>> this check.
>>>> 
>>>> In theory, the waring does not affect anything since the tail vmemmap
>>>> pages are supposed to be read-only. So, skipping this check for vmemmap
>>> 
>>> Tails vmemmap pages are supposed to be read-only, in practice but their
>>> backing pages do have pte_write() enabled. Otherwise the VM_WARN_ONCE()
>>> warning would not have triggered.
>> 
>> Right.
>> 
>>> 
>>>       VM_WARN_ONCE(pte_write(old_pte) && !pte_dirty(pte),
>>>                    "%s: racy dirty state clearing: 0x%016llx -> 0x%016llx",
>>>                    __func__, pte_val(old_pte), pte_val(pte));
>>> 
>>> Also, is not it true that the pte being remapped into a different page
>>> as read only, than what it had originally (which will be freed up) i.e 
>>> the PFN in 'old_pte' and 'pte' will be different. Hence is there still
>> 
>> Right.
>> 
>>> a possibility for a race condition even when the PFN changes ?
>> 
>> Sorry, I didn't get this question. Did you mean the PTE is changed from
>> new (pte) to the old one (old_pte) by the hardware because of the update
>> of dirty bit when a concurrent write operation to the tail vmemmap page?
> 
> No, but is not vmemmap_remap_pte() reuses walk->reuse_page for all remaining
> tails pages ? Is not there a PFN change, along with access permission change
> involved in this remapping process ?

Alright, yes, both the PFN and the access permission are changed.