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SubjectRe: [PATCH RFC v3 4/9] mm: Export alloc_contig_range() / free_contig_range()
On 16.10.19 13:20, Michal Hocko wrote:
> On Thu 19-09-19 16:22:23, David Hildenbrand wrote:
>> A virtio-mem device wants to allocate memory from the memory region it
>> manages in order to unplug it in the hypervisor - similar to
>> a balloon driver. Also, it might want to plug previously unplugged
>> (allocated) memory and give it back to Linux. alloc_contig_range() /
>> free_contig_range() seem to be the perfect interface for this task.
>> In contrast to existing balloon devices, a virtio-mem device operates
>> on bigger chunks (e.g., 4MB) and only on physical memory it manages. It
>> tracks which chunks (subblocks) are still plugged, so it can go ahead
>> and try to alloc_contig_range()+unplug them on unplug request, or
>> plug+free_contig_range() unplugged chunks on plug requests.
>> A virtio-mem device will use alloc_contig_range() / free_contig_range()
>> only on ranges that belong to the same node/zone in at least
>> MAX(MAX_ORDER - 1, pageblock_order) order granularity - e.g., 4MB on
>> x86-64. The virtio-mem device added that memory, so the memory
>> exists and does not contain any holes. virtio-mem will only try to allocate
>> on ZONE_NORMAL, never on ZONE_MOVABLE, just like when allocating
>> gigantic pages (we don't put unmovable data into the movable zone).
> Is there any real reason to export as GPL rather than generic
> EXPORT_SYMBOL? In other words do we need to restrict the usage this
> interface only to GPL modules and why if so. All other allocator APIs
> are EXPORT_SYMBOL so there should better be a good reason for this one
> to differ. I can understand that this one is slightly different by
> requesting a specific range of the memory but it is still under a full
> control of the core MM to say no.

I thought that we might - at least initially - might want to know all
users. If you prefer, I can drop the GPL.

>> Cc: Andrew Morton <>
>> Cc: Michal Hocko <>
>> Cc: Vlastimil Babka <>
>> Cc: Oscar Salvador <>
>> Cc: Mel Gorman <>
>> Cc: Mike Rapoport <>
>> Cc: Dan Williams <>
>> Cc: Alexander Duyck <>
>> Cc: Pavel Tatashin <>
>> Cc: Alexander Potapenko <>
>> Signed-off-by: David Hildenbrand <>
> Other than that, I do not think exporting this function is harmful. It
> would be worse to reinvent it and do it wrong.
> I usually prefer to add a caller in the same patch, though, because it
> makes the usage explicit and clear.

It's the next patch in this series (I prefer to split this from the
actual driver):

> Acked-by: Michal Hocko <> # to export contig range allocator API




David / dhildenb

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