From: David Hildenbrand <david@redhat.com>
To: Dan Williams <dan.j.williams@intel.com>
Cc: linux-hyperv@vger.kernel.org, "Michal Hocko" <mhocko@suse.com>,
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Subject: Re: [PATCH v1 01/10] mm/memory_hotplug: Don't allow to online/offline memory blocks with holes
Date: Tue, 5 Nov 2019 10:31:36 +0100 [thread overview]
Message-ID: <c52849c0-1185-c0c4-f9f6-2e7d12ed8e8e@redhat.com> (raw)
In-Reply-To: <CAPcyv4hT58=SDWYO2vrktdFOnDfWveVwN4ZBxNQ8=500_Zu7tQ@mail.gmail.com>
On 05.11.19 02:30, Dan Williams wrote:
> On Thu, Oct 24, 2019 at 5:10 AM David Hildenbrand <david@redhat.com> 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@linux-foundation.org>
>> Cc: Michal Hocko <mhocko@suse.com>
>> Cc: Oscar Salvador <osalvador@suse.de>
>> Cc: Pavel Tatashin <pasha.tatashin@soleen.com>
>> Cc: Dan Williams <dan.j.williams@intel.com>
>> Cc: Anshuman Khandual <anshuman.khandual@arm.com>
>> Signed-off-by: David Hildenbrand <david@redhat.com>
>> ---
>> 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@intel.com>
>
Thanks!
--
Thanks,
David / dhildenb
next prev parent reply other threads:[~2019-11-05 10:52 UTC|newest]
Thread overview: 43+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-10-24 12:09 [PATCH v1 00/10] mm: Don't mark hotplugged pages PG_reserved (including ZONE_DEVICE) David Hildenbrand
2019-10-24 12:09 ` [PATCH v1 01/10] mm/memory_hotplug: Don't allow to online/offline memory blocks with holes David Hildenbrand
2019-11-05 1:30 ` Dan Williams
2019-11-05 9:31 ` David Hildenbrand [this message]
2019-10-24 12:09 ` [PATCH v1 02/10] KVM: x86/mmu: Prepare kvm_is_mmio_pfn() for PG_reserved changes David Hildenbrand
2019-11-05 1:37 ` Dan Williams
2019-11-05 11:09 ` David Hildenbrand
2019-10-24 12:09 ` [PATCH v1 03/10] KVM: Prepare kvm_is_reserved_pfn() " David Hildenbrand
2019-11-05 4:38 ` Dan Williams
2019-11-05 9:17 ` David Hildenbrand
2019-11-05 9:49 ` David Hildenbrand
2019-11-05 10:02 ` David Hildenbrand
2019-11-05 16:00 ` Sean Christopherson
2019-11-05 20:30 ` David Hildenbrand
2019-11-05 22:22 ` Sean Christopherson
2019-11-05 23:02 ` Dan Williams
2019-11-05 23:13 ` Sean Christopherson
2019-11-05 23:30 ` Dan Williams
2019-11-05 23:42 ` Sean Christopherson
2019-11-05 23:43 ` Dan Williams
2019-11-06 0:03 ` Sean Christopherson
2019-11-06 0:08 ` Dan Williams
2019-11-06 6:56 ` David Hildenbrand
2019-11-06 16:09 ` Sean Christopherson
2019-10-24 12:09 ` [PATCH v1 04/10] vfio/type1: Prepare is_invalid_reserved_pfn() " David Hildenbrand
2019-11-07 15:40 ` Dan Williams
2019-11-07 18:22 ` David Hildenbrand
2019-11-07 22:07 ` David Hildenbrand
2019-11-08 5:09 ` Dan Williams
2019-11-08 7:14 ` David Hildenbrand
2019-11-08 10:21 ` David Hildenbrand
2019-11-08 18:29 ` Dan Williams
2019-11-08 23:01 ` David Hildenbrand
2019-10-24 12:09 ` [PATCH v1 05/10] powerpc/book3s: Prepare kvmppc_book3s_instantiate_page() " David Hildenbrand
2019-10-24 12:09 ` [PATCH v1 06/10] powerpc/64s: Prepare hash_page_do_lazy_icache() " David Hildenbrand
2019-10-24 12:09 ` [PATCH v1 07/10] powerpc/mm: Prepare maybe_pte_to_page() " David Hildenbrand
2019-10-24 12:09 ` [PATCH v1 08/10] x86/mm: Prepare __ioremap_check_ram() " David Hildenbrand
2019-10-24 12:09 ` [PATCH v1 09/10] mm/memory_hotplug: Don't mark pages PG_reserved when initializing the memmap David Hildenbrand
2019-11-04 22:44 ` Boris Ostrovsky
2019-11-05 10:18 ` David Hildenbrand
2019-11-05 16:06 ` Boris Ostrovsky
2019-10-24 12:09 ` [PATCH v1 10/10] mm/usercopy.c: Update comment in check_page_span() regarding ZONE_DEVICE David Hildenbrand
2019-11-01 19:24 ` [PATCH v1 00/10] mm: Don't mark hotplugged pages PG_reserved (including ZONE_DEVICE) David Hildenbrand
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