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SubjectRe: [External] Re: [PATCH v16 4/9] mm: hugetlb: alloc the vmemmap pages associated with each HugeTLB page
On Tue, Feb 23, 2021 at 11:50:05AM +0100, Oscar Salvador wrote:
> > CPU0: CPU1:
> > set_compound_page_dtor(HUGETLB_PAGE_DTOR);
> > memory_failure_hugetlb
> > get_hwpoison_page
> > __get_hwpoison_page
> > get_page_unless_zero
> > put_page_testzero()
> >
> > Maybe this can happen. But it is a very corner case. If we want to
> > deal with this. We can put_page_testzero() first and then
> > set_compound_page_dtor(HUGETLB_PAGE_DTOR).
>
> I have to check further, but it looks like this could actually happen.
> Handling this with VM_BUG_ON is wrong, because memory_failure/soft_offline are
> entitled to increase the refcount of the page.
>
> AFAICS,
>
> CPU0: CPU1:
> set_compound_page_dtor(HUGETLB_PAGE_DTOR);
> memory_failure_hugetlb
> get_hwpoison_page
> __get_hwpoison_page
> get_page_unless_zero
> put_page_testzero()
> identify_page_state
> me_huge_page
>
> I think we can reach me_huge_page with either refcount = 1 or refcount =2,
> depending whether put_page_testzero has been issued.
>
> For now, I would not re-enqueue the page if put_page_testzero == false.
> I have to see how this can be handled gracefully.

I took a brief look.
It is not really your patch fault. Hugetlb <-> memory-failure synchronization is
a bit odd, it definitely needs improvment.

The thing is, we can have different scenarios here.
E.g: by the time we return from put_page_testzero, we might have refcount ==
0 and PageHWPoison, or refcount == 1 PageHWPoison.

The former will let a user get a page from the pool and get a sigbus
when it faults in the page, and the latter will be even more odd as we
will have a self-refcounted page in the free pool (and hwpoisoned).

As I said, it is not this patchset fault. I just made me realize this
problem.

I have to think some more about this.

--
Oscar Salvador
SUSE L3

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