Re: [External] Re: [PATCH v18 5/9] mm: hugetlb: set the PageHWPoison to the raw error page

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On Thu 11-03-21 14:34:04, Muchun Song wrote:
> On Wed, Mar 10, 2021 at 11:28 PM Michal Hocko <mhocko@xxxxxxxx> wrote:
> >
> > On Mon 08-03-21 18:28:03, Muchun Song wrote:
> > > Because we reuse the first tail vmemmap page frame and remap it
> > > with read-only, we cannot set the PageHWPosion on some tail pages.
> > > So we can use the head[4].private (There are at least 128 struct
> > > page structures associated with the optimized HugeTLB page, so
> > > using head[4].private is safe) to record the real error page index
> > > and set the raw error page PageHWPoison later.
> >
> > Can we have more poisoned tail pages? Also who does consume that index
> > and set the HWPoison on the proper tail page?
> 
> Good point. I look at the routine of memory failure closely.
> If we do not clear the HWPoison of the head page, we cannot
> poison another tail page.
> 
> So we should not set the destructor of the huge page from
> HUGETLB_PAGE_DTOR to NULL_COMPOUND_DTOR
> before calling alloc_huge_page_vmemmap(). In this case,
> the below check of PageHuge() always returns true.
> 
> I need to fix this in the previous patch.
> 
> memory_failure()
>     if (PageHuge(page))
>         memory_failure_hugetlb()
>             head = compound_head(page)
>             if (TestSetPageHWPoison(head))
>                 return

I have to say that I am not fully familiar with hwpoisoning code
(especially after recent changes) but IIRC it does rely on hugetlb page
dissolving. With the new code this operation can fail which is a new
situation. Unless I am misunderstanding this can lead to a lost memory
failure operation on other tail pages.

Anyway the above answers the question why a single slot is sufficient so
it would be great to mention that in a changelog along with the caveat
that some pages might miss their poisoning.
-- 
Michal Hocko
SUSE Labs




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