Re: [RFC PATCH v1 0/4] mm, hwpoison: improve handling workload related to hugetlb and memory_hotplug

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

On Wed, May 11, 2022 at 06:22:41PM +0200, David Hildenbrand wrote:
> On 11.05.22 18:10, HORIGUCHI NAOYA(堀口 直也) wrote:
> > On Wed, May 11, 2022 at 05:11:17PM +0200, David Hildenbrand wrote:
> >> On 09.05.22 12:53, Miaohe Lin wrote:
> >>> On 2022/5/9 17:58, Oscar Salvador wrote:
> >>>> On Mon, May 09, 2022 at 05:04:54PM +0800, Miaohe Lin wrote:
> >>>>>>> So that leaves us with either
> >>>>>>>
> >>>>>>> 1) Fail offlining -> no need to care about reonlining
> >>>>>
> >>>>> Maybe fail offlining will be a better alternative as we can get rid of many races
> >>>>> between memory failure and memory offline? But no strong opinion. :)
> >>>>
> >>>> If taking care of those races is not an herculean effort, I'd go with
> >>>> allowing offlining + disallow re-onlining.
> >>>> Mainly because memory RAS stuff.
> >>>
> >>> This dose make sense to me. Thanks. We can try to solve those races if
> >>> offlining + disallow re-onlining is applied. :)
> >>>
> >>>>
> >>>> Now, to the re-onlining thing, we'll have to come up with a way to check
> >>>> whether a section contains hwpoisoned pages, so we do not have to go
> >>>> and check every single page, as that will be really suboptimal.
> >>>
> >>> Yes, we need a stable and cheap way to do that.
> >>
> >> My simplistic approach would be a simple flag/indicator in the memory block devices
> >> that indicates that any page in the memory block was hwpoisoned. It's easy to
> >> check that during memory onlining and fail it.
> >>
> >> diff --git a/drivers/base/memory.c b/drivers/base/memory.c
> >> index 084d67fd55cc..3d0ef812e901 100644
> >> --- a/drivers/base/memory.c
> >> +++ b/drivers/base/memory.c
> >> @@ -183,6 +183,9 @@ static int memory_block_online(struct memory_block *mem)
> >>         struct zone *zone;
> >>         int ret;
> >>  
> >> +       if (mem->hwpoisoned)
> >> +               return -EHWPOISON;
> >> +
> >>         zone = zone_for_pfn_range(mem->online_type, mem->nid, mem->group,
> >>                                   start_pfn, nr_pages);
> >>  
> > 
> > Thanks for the idea, a simple flag could work if we don't have to consider
> > unpoison.  If we need consider unpoison, we need remember the last hwpoison
> > page in the memory block, so mem->hwpoisoned should be the counter of
> > hwpoison pages.
> 
> Right, but unpoisoning+memory offlining+memory onlining is a yet more
> extreme use case we don't have to bother about I think.

OK. Maybe starting with simple one is fine.

> 
> > 
> >>
> >>
> >> Once the problematic DIMM would actually get unplugged, the memory block devices
> >> would get removed as well. So when hotplugging a new DIMM in the same
> >> location, we could online that memory again.
> > 
> > What about PG_hwpoison flags?  struct pages are also freed and reallocated
> > in the actual DIMM replacement?
> 
> Once memory is offline, the memmap is stale and is no longer
> trustworthy. It gets reinitialize during memory onlining -- so any
> previous PG_hwpoison is overridden at least there. In some setups, we
> even poison the whole memmap via page_init_poison() during memory offlining.
> 
> Apart from that, we should be freeing the memmap in all relevant cases
> when removing memory. I remember there are a couple of corner cases, but
> we don't really have to care about that.

OK, so there seems no need to manipulate struct pages for hwpoison in
all relevant cases.

Thanks,
Naoya Horiguchi