Re: [PATCH v1 1/5] mm, hwpoison, hugetlb: introduce SUBPAGE_INDEX_HWPOISON to save raw error page

[David Hildenbrand]

On 08.06.22 03:31, HORIGUCHI NAOYA(堀口 直也) wrote:
> On Tue, Jun 07, 2022 at 03:04:15PM +0200, David Hildenbrand wrote:
>> On 02.06.22 07:06, Naoya Horiguchi wrote:
>>> From: Naoya Horiguchi <naoya.horiguchi@xxxxxxx>
>>>
>>> When handling memory error on a hugetlb page, the error handler tries to
>>> dissolve and turn it into 4kB pages.  If it's successfully dissolved,
>>> PageHWPoison flag is moved to the raw error page, so that's all right.
>>> However, dissolve sometimes fails, then the error page is left as
>>> hwpoisoned hugepage. It's useful if we can retry to dissolve it to save
>>> healthy pages, but that's not possible now because the information about
>>> where the raw error page is lost.
>>>
>>> Use the private field of a tail page to keep that information.  The code
>>> path of shrinking hugepage pool used this info to try delayed dissolve.
>>> This only keeps one hwpoison page for now, which might be OK because it's
>>> simple and multiple hwpoison pages in a hugepage can be rare. But it can
>>> be extended in the future.
>>>
>>>
>>
>> But what would happen now if you have multiple successive MCE events on
>> such a page now?
> 
> The 2nd and later events are ignored due to "already hwpoisoned hugepage",
> this might not be good when the hwpoisoned hugepage is freed/dissolved later.
> So a temporal workaround is to remember "hugepage has multiple hwpoison pages"
> and disable free/dissolve for such hugepages.

Right. We might want to indicate exactly one vs. multiple, and then in
__update_and_free_page(), move the hwpoison flag to one subpage,
eventually exposing other corrupted subpages to the system.

-- 
Thanks,

David / dhildenb