Re: [External] Re: [PATCH v16 4/9] mm: hugetlb: alloc the vmemmap pages associated with each HugeTLB page

[Oscar Salvador <osalvador@suse.de>]

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