Re: [Xen-devel] [PATCH v1 01/10] mm/memory_hotplug: Don't allow to online/offline memory blocks with holes

[David Hildenbrand <david@xxxxxxxxxx>]

On 05.11.19 02:30, Dan Williams wrote:
> On Thu, Oct 24, 2019 at 5:10 AM David Hildenbrand <david@xxxxxxxxxx> wrote:
> > Our onlining/offlining code is unnecessarily complicated. Only memory
> > blocks added during boot can have holes (a range that is not
> > IORESOURCE_SYSTEM_RAM). Hotplugged memory never has holes (e.g., see
> > add_memory_resource()). All boot memory is alread online.
> s/alread/already/
Thanks.

> ...also perhaps clarify "already online" by what point in time and why
> that is relevant. For example a description of the difference between
> the SetPageReserved() in the bootmem path and the one in the hotplug
> path.
Will add.

> > Therefore, when we stop allowing to offline memory blocks with holes, we
> > implicitly no longer have to deal with onlining memory blocks with holes.
> Maybe an explicit reference of the code areas that deal with holes
> would help to back up that assertion. Certainly it would have saved me
> some time for the review.
I can add a reference to the onlining code that will only online pages 
that don't fall into memory holes.
> > This allows to simplify the code. For example, we no longer have to
> > worry about marking pages that fall into memory holes PG_reserved when
> > onlining memory. We can stop setting pages PG_reserved.
> ...but not for bootmem, right?
Yes, bootmem is not changed. (especially, early allocations and memory 
holes are marked PG_reserved - basically everything not given to the 
buddy after boot)
> > Offlining memory blocks added during boot is usually not guranteed to work
> s/guranteed/guaranteed/
Thanks.

> > either way (unmovable data might have easily ended up on that memory during
> > boot). So stopping to do that should not really hurt (+ people are not
> > even aware of a setup where that used to work
> Maybe put a "Link: https://lkml.kernel.org/r/$msg_id"; to that discussion?
>
> > and that the existing code
> > still works correctly with memory holes). For the use case of offlining
> > memory to unplug DIMMs, we should see no change. (holes on DIMMs would be
> > weird).
> However, less memory can be offlined than was theoretically allowed
> previously, so I don't understand the "we should see no change"
> comment. I still agree that's a price worth paying to get the code
> cleanups and if someone screams we can look at adding it back, but the
> fact that it was already fragile seems decent enough protection.
Hotplugged DIMMs (add_memory()) have no holes and will therefore see no 
change.
> > Please note that hardware errors (PG_hwpoison) are not memory holes and
> > not affected by this change when offlining.
> >
> > Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> > Cc: Michal Hocko <mhocko@xxxxxxxx>
> > Cc: Oscar Salvador <osalvador@xxxxxxx>
> > Cc: Pavel Tatashin <pasha.tatashin@xxxxxxxxxx>
> > Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
> > Cc: Anshuman Khandual <anshuman.khandual@xxxxxxx>
> > Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
> > ---
> >   mm/memory_hotplug.c | 26 ++++++++++++++++++++++++--
> >   1 file changed, 24 insertions(+), 2 deletions(-)
> >
> > diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> > index 561371ead39a..8d81730cf036 100644
> > --- a/mm/memory_hotplug.c
> > +++ b/mm/memory_hotplug.c
> > @@ -1447,10 +1447,19 @@ static void node_states_clear_node(int node, struct 
> > memory_notify *arg)
> >                  node_clear_state(node, N_MEMORY);
> >   }
> >
> > +static int count_system_ram_pages_cb(unsigned long start_pfn,
> > +                                    unsigned long nr_pages, void *data)
> > +{
> > +       unsigned long *nr_system_ram_pages = data;
> > +
> > +       *nr_system_ram_pages += nr_pages;
> > +       return 0;
> > +}
> > +
> >   static int __ref __offline_pages(unsigned long start_pfn,
> >                    unsigned long end_pfn)
> >   {
> > -       unsigned long pfn, nr_pages;
> > +       unsigned long pfn, nr_pages = 0;
> >          unsigned long offlined_pages = 0;
> >          int ret, node, nr_isolate_pageblock;
> >          unsigned long flags;
> > @@ -1461,6 +1470,20 @@ static int __ref __offline_pages(unsigned long start_pfn,
> >
> >          mem_hotplug_begin();
> >
> > +       /*
> > +        * Don't allow to offline memory blocks that contain holes.
> > +        * Consecuently, memory blocks with holes can never get onlined
> s/Consecuently/Consequently/
Thanks.

> > +        * (hotplugged memory has no holes and all boot memory is online).
> > +        * This allows to simplify the onlining/offlining code quite a lot.
> > +        */
> The last sentence of this comment makes sense in the context of this
> patch, but I don't think it stands by itself in the code base after
> the fact. The person reading the comment can't see the simplifications
> because the code is already gone. I'd clarify it to talk about why it
> is safe to not mess around with PG_Reserved in the hotplug path
> because of this check.
I'll think of something. It's not just the PG_reserved handling but the 
whole pfn_valid()/walk_system_ram_range() handling that can be simplified.
> After those clarifications you can add:
>
> Reviewed-by: Dan Williams <dan.j.williams@xxxxxxxxx>
Thanks!

--

Thanks,

David / dhildenb


_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxxx
https://lists.xenproject.org/mailman/listinfo/xen-devel