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

[HORIGUCHI NAOYA(堀口 直也)]

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.

Thanks,
Naoya Horiguchi