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

[Muchun Song <songmuchun@bytedance.com>]

On Wed, Feb 24, 2021 at 6:32 AM Oscar Salvador <osalvador@suse.de> wrote:
>
> On Tue, Feb 23, 2021 at 04:41:28PM +0100, Oscar Salvador wrote:
> > 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).

I have been looking at the dequeue_huge_page_node_exact().
If a PageHWPoison huge page is in the free pool list, the page will
not be allocated to the user. The PageHWPoison huge page
will be skip in the dequeue_huge_page_node_exact().

> >
> > As I said, it is not this patchset fault. I just made me realize this
> > problem.
> >
> > I have to think some more about this.
>
> I have been thinking more about this.
> memory failure events can occur at any time, and we might not be in a
> position where we can handle gracefully the error, meaning that the page
> might end up in non desirable state.
>
> E.g: we could flag the page right before enqueing it.
>
> I still think that VM_BUG_ON should go, as the refcount can be perfectly
> increased by memory-failure/soft_offline handlers, so BUGing there does
> not make much sense.

Make sense. I will remove the VM_BUG_ON.

>
> One think we could do is to check the state of the page we want to
> retrieve from the free hugepage pool.
> We should discard any HWpoisoned ones, and dissolve them.
>
> The thing is, memory-failure/soft_offline should allocate a new hugepage
> for the free pool, so keep the pool stable.
> Something like [1].
>
> Anyway, this is orthogonal to this patch, and something I will work on
> soon.
>
> [1] https://lore.kernel.org/linux-mm/20210222135137.25717-2-osalvador@suse.de/T/#u

Thanks for your efforts on this.

>
> --
> Oscar Salvador
> SUSE L3