Re: [PATCH RFC v2 19/27] mm: mprotect: Introduce PAGE_FAULT_ON_ACCESS for mprotect(PROT_MTE)

David Hildenbrand
2023-11-30T13:44:25Z

On 30.11.23 14:32, Alexandru Elisei wrote:
> Hi,
>
> On Thu, Nov 30, 2023 at 01:49:34PM +0100, David Hildenbrand wrote:
>>>>> +
>>>>> +out_retry:
>>>>> + put_page(page);
>>>>> + if (vmf->flags & FAULT_FLAG_VMA_LOCK)
>>>>> + vma_end_read(vma);
>>>>> + if (fault_flag_allow_retry_first(vmf->flags)) {
>>>>> + err = VM_FAULT_RETRY;
>>>>> + } else {
>>>>> + /* Replay the fault. */
>>>>> + err = 0;
>>>>
>>>> Hello!
>>>>
>>>> Unfortunately, if the page continues to be pinned, it seems like fault will continue to occur.
>>>> I guess it makes system stability issue. (but I'm not familiar with that, so please let me know if I'm mistaken!)
>>>>
>>>> How about migrating the page when migration problem repeats.
>>>
>>> Yes, I had the same though in the previous iteration of the series, the
>>> page was migrated out of the VMA if tag storage couldn't be reserved.
>>>
>>> Only short term pins are allowed on MIGRATE_CMA pages, so I expect that the
>>> pin will be released before the fault is replayed. Because of this, and
>>> because it makes the code simpler, I chose not to migrate the page if tag
>>> storage couldn't be reserved.
>>
>> There are still some cases that are theoretically problematic: vmsplice()
>> can pin pages forever and doesn't use FOLL_LONGTERM yet.
>>
>> All these things also affect other users that rely on movability (e.g., CMA,
>> memory hotunplug).
>
> I wasn't aware of that, thank you for the information. Then to ensure that the
> process doesn't hang by replying the loop indefinitely, I'll migrate the page if
> tag storage cannot be reserved. Looking over the code again, I think I can reuse
> the same function that migrates tag storage pages out of the MTE VMA (added in
> patch #21), so no major changes needed.

It's going to be interesting if migrating that page fails because it is
pinned :/

--
Cheers,

David / dhildenb