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 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.

> 
> 
> 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?

Thanks,
Naoya Horiguchi

> 
> Another place to store that would be the memory section, we'd then have to check
> all underlying sections here.
> 
> We're a bit short on flags in the memory section I think, but they are easier to
> lookup from other code eventually then memory block devices.