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Date: Mon, 10 Jun 2024 10:56:02 +0200
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Subject: Re: [PATCH v1 2/3] mm/memory_hotplug: initialize memmap of
 !ZONE_DEVICE with PageOffline() instead of PageReserved()
To: Oscar Salvador <osalvador@suse.de>
Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org,
 linux-hyperv@vger.kernel.org, virtualization@lists.linux.dev,
 xen-devel@lists.xenproject.org, kasan-dev@googlegroups.com,
 Andrew Morton <akpm@linux-foundation.org>, Mike Rapoport <rppt@kernel.org>,
 "K. Y. Srinivasan" <kys@microsoft.com>,
 Haiyang Zhang <haiyangz@microsoft.com>, Wei Liu <wei.liu@kernel.org>,
 Dexuan Cui <decui@microsoft.com>, "Michael S. Tsirkin" <mst@redhat.com>,
 Jason Wang <jasowang@redhat.com>, Xuan Zhuo <xuanzhuo@linux.alibaba.com>,
 =?UTF-8?Q?Eugenio_P=C3=A9rez?= <eperezma@redhat.com>,
 Juergen Gross <jgross@suse.com>, Stefano Stabellini
 <sstabellini@kernel.org>,
 Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>,
 Alexander Potapenko <glider@google.com>, Marco Elver <elver@google.com>,
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References: <20240607090939.89524-1-david@redhat.com>
 <20240607090939.89524-3-david@redhat.com>
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From: David Hildenbrand <david@redhat.com>
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On 10.06.24 06:23, Oscar Salvador wrote:
> On Fri, Jun 07, 2024 at 11:09:37AM +0200, David Hildenbrand wrote:
>> We currently initialize the memmap such that PG_reserved is set and the
>> refcount of the page is 1. In virtio-mem code, we have to manually clear
>> that PG_reserved flag to make memory offlining with partially hotplugged
>> memory blocks possible: has_unmovable_pages() would otherwise bail out on
>> such pages.
>>
>> We want to avoid PG_reserved where possible and move to typed pages
>> instead. Further, we want to further enlighten memory offlining code about
>> PG_offline: offline pages in an online memory section. One example is
>> handling managed page count adjustments in a cleaner way during memory
>> offlining.
>>
>> So let's initialize the pages with PG_offline instead of PG_reserved.
>> generic_online_page()->__free_pages_core() will now clear that flag before
>> handing that memory to the buddy.
>>
>> Note that the page refcount is still 1 and would forbid offlining of such
>> memory except when special care is take during GOING_OFFLINE as
>> currently only implemented by virtio-mem.
>>
>> With this change, we can now get non-PageReserved() pages in the XEN
>> balloon list. From what I can tell, that can already happen via
>> decrease_reservation(), so that should be fine.
>>
>> HV-balloon should not really observe a change: partial online memory
>> blocks still cannot get surprise-offlined, because the refcount of these
>> PageOffline() pages is 1.
>>
>> Update virtio-mem, HV-balloon and XEN-balloon code to be aware that
>> hotplugged pages are now PageOffline() instead of PageReserved() before
>> they are handed over to the buddy.
>>
>> We'll leave the ZONE_DEVICE case alone for now.
>>
>> Signed-off-by: David Hildenbrand <david@redhat.com>
> 
>> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
>> index 27e3be75edcf7..0254059efcbe1 100644
>> --- a/mm/memory_hotplug.c
>> +++ b/mm/memory_hotplug.c
>> @@ -734,7 +734,7 @@ static inline void section_taint_zone_device(unsigned long pfn)
>>   /*
>>    * Associate the pfn range with the given zone, initializing the memmaps
>>    * and resizing the pgdat/zone data to span the added pages. After this
>> - * call, all affected pages are PG_reserved.
>> + * call, all affected pages are PageOffline().
>>    *
>>    * All aligned pageblocks are initialized to the specified migratetype
>>    * (usually MIGRATE_MOVABLE). Besides setting the migratetype, no related
>> @@ -1100,8 +1100,12 @@ int mhp_init_memmap_on_memory(unsigned long pfn, unsigned long nr_pages,
>>   
>>   	move_pfn_range_to_zone(zone, pfn, nr_pages, NULL, MIGRATE_UNMOVABLE);
>>   
>> -	for (i = 0; i < nr_pages; i++)
>> -		SetPageVmemmapSelfHosted(pfn_to_page(pfn + i));
>> +	for (i = 0; i < nr_pages; i++) {
>> +		struct page *page = pfn_to_page(pfn + i);
>> +
>> +		__ClearPageOffline(page);
>> +		SetPageVmemmapSelfHosted(page);
> 
> So, refresh my memory here please.
> AFAIR, those VmemmapSelfHosted pages were marked Reserved before, but now,
> memmap_init_range() will not mark them reserved anymore.

Correct.

> I do not think that is ok? I am worried about walkers getting this wrong.
> 
> We usually skip PageReserved pages in walkers because are pages we cannot deal
> with for those purposes, but with this change, we will leak
> PageVmemmapSelfHosted, and I am not sure whether are ready for that.

There are fortunately not that many left.

I'd even say marking them (vmemmap) reserved is more wrong than right: 
note that ordinary vmemmap pages after memory hotplug are not reserved! 
Only bootmem should be reserved.

Let's take at the relevant core-mm ones (arch stuff is mostly just for 
MMIO remapping)

fs/proc/task_mmu.c:     if (PageReserved(page))
fs/proc/task_mmu.c:     if (PageReserved(page))

-> If we find vmemmap pages mapped into user space we already messed up
    seriously

kernel/power/snapshot.c:        if (PageReserved(page) ||
kernel/power/snapshot.c:        if (PageReserved(page)

-> There should be no change (saveable_page() would still allow saving
    them, highmem does not apply)

mm/hugetlb_vmemmap.c:           if (!PageReserved(head))
mm/hugetlb_vmemmap.c:   if (PageReserved(page))

-> Wants to identify bootmem, but we exclude these
    PageVmemmapSelfHosted() on the splitting part already properly


mm/page_alloc.c:                VM_WARN_ON_ONCE(PageReserved(p));
mm/page_alloc.c:                if (PageReserved(page))

-> pfn_range_valid_contig() would scan them, just like for ordinary
    vmemmap pages during hotplug. We'll simply fail isolating/migrating
    them similarly (like any unmovable allocations) later

mm/page_ext.c:          BUG_ON(PageReserved(page));

-> free_page_ext handling, does not apply

mm/page_isolation.c:            if (PageReserved(page))

-> has_unmovable_pages() should still detect them as unmovable (e.g.,
    neither movable nor LRU).

mm/page_owner.c:                        if (PageReserved(page))
mm/page_owner.c:                        if (PageReserved(page))

-> Simply page_ext_get() will return NULL instead and we'll similarly
    skip them

mm/sparse.c:            if (!PageReserved(virt_to_page(ms->usage))) {

-> Detecting boot memory for ms->usage allocation, does not apply to
    vmemmap.

virt/kvm/kvm_main.c:    if (!PageReserved(page))
virt/kvm/kvm_main.c:    return !PageReserved(page);

-> For MMIO remapping purposes, does not apply to vmemmap


> Moreover, boot memmap pages are marked as PageReserved, which would be
> now inconsistent with those added during hotplug operations.

Just like vmemmap pages allocated dynamically during memory hotplug. 
Now, really only bootmem-ones are PageReserved.

> All in all, I feel uneasy about this change.

I really don't want to mark these pages here PageReserved for the sake 
of it.

Any PageReserved user that I am missing, or why we should handle these 
vmemmap pages differently than the ones allocated during ordinary memory 
hotplug?

In the future, we might want to consider using a dedicated page type for 
them, so we can stop using a bit that doesn't allow to reliably identify 
them. (we should mark all vmemmap with that type then)

Thanks!

-- 
Cheers,

David / dhildenb