Re: [RFC PATCH v1 0/3] Userspace MFR Policy via memfd

[Miaohe Lin]

On 2025/11/4 0:57, Jiaqi Yan wrote:
> On Mon, Nov 3, 2025 at 12:53 AM Harry Yoo <harry.yoo@xxxxxxxxxx> wrote:
>>
>> On Mon, Nov 03, 2025 at 05:16:33PM +0900, Harry Yoo wrote:
>>> On Thu, Oct 30, 2025 at 10:28:48AM -0700, Jiaqi Yan wrote:
>>>> On Thu, Oct 30, 2025 at 4:51 AM Miaohe Lin <linmiaohe@xxxxxxxxxx> wrote:
>>>>> On 2025/10/28 15:00, Harry Yoo wrote:
>>>>>> On Mon, Oct 27, 2025 at 09:17:31PM -0700, Jiaqi Yan wrote:
>>>>>>> On Wed, Oct 22, 2025 at 6:09 AM Harry Yoo <harry.yoo@xxxxxxxxxx> wrote:
>>>>>>>> On Mon, Oct 13, 2025 at 03:14:32PM -0700, Jiaqi Yan wrote:
>>>>>>>>> On Fri, Sep 19, 2025 at 8:58 AM “William Roche <william.roche@xxxxxxxxxx> wrote:
>>>>>>>> But even after fixing that we need to fix the race condition.
>>>>>>>
>>>>>>> What exactly is the race condition you are referring to?
>>>>>>
>>>>>> When you free a high-order page, the buddy allocator doesn't not check
>>>>>> PageHWPoison() on the page and its subpages. It checks PageHWPoison()
>>>>>> only when you free a base (order-0) page, see free_pages_prepare().
>>>>>
>>>>> I think we might could check PageHWPoison() for subpages as what free_page_is_bad()
>>>>> does. If any subpage has HWPoisoned flag set, simply drop the folio. Even we could
>>>>
>>>> Agree, I think as a starter I could try to, for example, let
>>>> free_pages_prepare scan HWPoison-ed subpages if the base page is high
>>>> order. In the optimal case, HugeTLB does move PageHWPoison flag from
>>>> head page to the raw error pages.
>>>
>>> [+Cc page allocator folks]
>>>
>>> AFAICT enabling page sanity check in page alloc/free path would be against
>>> past efforts to reduce sanity check overhead.
>>>
>>> [1] https://lore.kernel.org/linux-mm/1460711275-1130-15-git-send-email-mgorman@xxxxxxxxxxxxxxxxxxx/
>>> [2] https://lore.kernel.org/linux-mm/1460711275-1130-16-git-send-email-mgorman@xxxxxxxxxxxxxxxxxxx/
>>> [3] https://lore.kernel.org/all/20230216095131.17336-1-vbabka@xxxxxxx
>>>
>>> I'd recommend to check hwpoison flag before freeing it to the buddy
>>> when we know a memory error has occurred (I guess that's also what Miaohe
>>> suggested).
>>>
>>>>> do it better -- Split the folio and let healthy subpages join the buddy while reject
>>>>> the hwpoisoned one.
>>>>>
>>>>>>
>>>>>> AFAICT there is nothing that prevents the poisoned page to be
>>>>>> allocated back to users because the buddy doesn't check PageHWPoison()
>>>>>> on allocation as well (by default).
>>>>>>
>>>>>> So rather than freeing the high-order page as-is in
>>>>>> dissolve_free_hugetlb_folio(), I think we have to split it to base pages
>>>>>> and then free them one by one.
>>>>>
>>>>> It might not be worth to do that as this would significantly increase the overhead
>>>>> of the function while memory failure event is really rare.
>>>>
>>>> IIUC, Harry's idea is to do the split in dissolve_free_hugetlb_folio
>>>> only if folio is HWPoison-ed, similar to what Miaohe suggested
>>>> earlier.
>>>
>>> Yes, and if we do the check before moving HWPoison flag to raw pages,
>>> it'll be just a single folio_test_hwpoison() call.
>>>
>>>> BTW, I believe this race condition already exists today when
>>>> memory_failure handles HWPoison-ed free hugetlb page; it is not
>>>> something introduced via this patchset. I will fix or improve this in
>>>> a separate patchset.
>>>
>>> That makes sense.
>>
>> Wait, without this patchset, do we even free the hugetlb folio when
>> its subpage is hwpoisoned? I don't think we do, but I'm not expert at MFR...
> 
> Based on my reading of try_memory_failure_hugetlb, me_huge_page, and
> __page_handle_poison, I think mainline kernel frees dissolved hugetlb
> folio to buddy allocator in two cases:
> 1. it was a free hugetlb page at the moment of try_memory_failure_hugetlb
> 2. it was an anonomous hugetlb page

I think there are some corner cases that can lead to hugetlb folio being freed while
some of its subpages are hwpoisoned. E.g. get_huge_page_for_hwpoison can return
-EHWPOISON when hugetlb folio is happen to be isolated. Later hugetlb folio might
become free and __update_and_free_hugetlb_folio will be used to free it into buddy.

If page sanity check is enabled, hwpoisoned subpages will slip into buddy but they
won't be re-allocated later because check_new_page will drop them. But if page sanity
check is disabled, I think there is still missing a way to stop hwpoisoned subpages
from being reused.

Let me know if I miss something.

Thanks both.
.