Re: [RFC PATCH v1 2/4] mm,hwpoison,hugetlb,memory_hotplug: hotremove memory section with hwpoisoned hugepage

["HORIGUCHI NAOYA(堀口 直也)" <naoya.horiguchi@nec.com>]

On Fri, Apr 29, 2022 at 04:49:16PM +0800, Miaohe Lin wrote:
> On 2022/4/27 12:28, Naoya Horiguchi wrote:
> > From: Naoya Horiguchi <naoya.horiguchi@nec.com>
> > 
> > HWPoisoned page is not supposed to prevent memory hotremove, but
> > currently this does not properly work for hwpoisoned hugepages and the
> > kernel tries to migrate them, which could cause consuming corrupted
> > data.
> > 
> > Move dissolve_free_huge_pages() before scan_movable_pages(). This is
> > because the result of the movable check depends on the result of the
> > dissolve.  Now delayed dissolve is available, so hwpoisoned hugepages
> > can be turned into 4kB hwpoison page which memory hotplug can handle.
> > 
> > And clear HPageMigratable pseudo flag for hwpoisoned hugepages. This is
> > also important because dissolve_free_huge_page() can fail.  So it's
> > still necessary to prevent do_migrate_pages() from trying to migrate
> > hwpoison hugepages.
> > 
> > Reported-by: Miaohe Lin <linmiaohe@huawei.com>
> > Signed-off-by: Naoya Horiguchi <naoya.horiguchi@nec.com>
> > ---
> >  mm/hugetlb.c        | 11 +++++++++++
> >  mm/memory-failure.c |  2 ++
> >  mm/memory_hotplug.c | 23 +++++++++++------------
> >  3 files changed, 24 insertions(+), 12 deletions(-)
> > 
> > diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> > index 6867ea8345d1..95b1db852ca9 100644
> > --- a/mm/hugetlb.c
> > +++ b/mm/hugetlb.c
> > @@ -2159,6 +2159,17 @@ int dissolve_free_huge_pages(unsigned long start_pfn, unsigned long end_pfn)
> >  
> >  	for (pfn = start_pfn; pfn < end_pfn; pfn += 1 << minimum_order) {
> >  		page = pfn_to_page(pfn);
> > +
> > +		if (PageHuge(page) && PageHWPoison(page)) {
> > +			/*
> > +			 * Release the last refcount from hwpoison to turn into
> > +			 * a free hugepage.
> > +			 */
> > +			if (page_count(page) == 1)
> > +				put_page(page);
> > +			page = hugetlb_page_hwpoison(page);
> > +		}
> > +
> 
> This patch looks good to me. Thanks!
> 
> One question: Can this hugepage be put into buddy system? In free_huge_page,
> if h->surplus_huge_pages_node[nid] > 0, hugepage might be put into buddy via
> update_and_free_page. So it's not PageHuge anymore and won't be dissolved. If
> this happens, the "raw error page" is still missed and might be accessed later.

Yes, this put_page() could free pages directly into buddy.  In such case, I
expect __update_and_free_page() to move the PageHWpoison flag to the raw error
page, so I think the final result should be the same.

Thanks,
Naoya Horiguchi