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

[Oscar Salvador]

On Mon, Feb 22, 2021 at 06:31:12PM +0800, Muchun Song wrote:
> On Mon, Feb 22, 2021 at 5:25 PM Michal Hocko <mhocko@xxxxxxxx> wrote:
> >
> > On Sat 20-02-21 12:20:36, Muchun Song wrote:
> > > On Fri, Feb 19, 2021 at 10:12 PM Michal Hocko <mhocko@xxxxxxxx> wrote:
> > [...]
> > > > What about hugetlb page poisoning on HW failure (resp. soft offlining)?
> > >
> > > If the HW poisoned hugetlb page failed to be dissolved, the page
> > > will go back to the free list with PG_HWPoison set. But the page
> > > will not be used, because we will check whether the page is HW
> > > poisoned when it is dequeued from the free list. If so, we will skip
> > > this page.

Not really. If the huge page is dissolved, we will take the page out of the
the freelist. See take_page_off_buddy in memory_failure_hugetlb.

In an ideal world, we should inspect that page in free_pages_prepare(),
remove the HPWpoisoned page and process the others, without letting that
page hit Buddy.
And not only for hugetlb, but for any higher order page.
See how memory_failure() happily disengage itself when it finds a higher
order page.
It does it because we have the premise that once that page hits Buddy,
it will stay there as the check_new_page guards us.
But this has been proofed to be quite a weak measure, as compaction does
not performs such a check, and so the page can sneak in.

I fixed that for soft-offline, and for memory-failure in some cases, but more
needs to be done and is it in my TODO list.

> > Can this lead to an under provisioned pool then? Or is there a new
> > hugetlb allocated to replace the poisoned one?
> 
> Actually, no page will be allocated. Your concern is right. But without
> this patch, the result does not change. e.g. The HW poisoned page
> can fail to be dissolved when h->free_huge_pages is equal to
> h->resv_huge_pages. But no one seems to have reported this issue so
> far. Maybe this behavior needs improvement in the feature.

Yes, something to improve.
I shall have a look.

-- 
Oscar Salvador
SUSE L3