Re: [PATCH RFC v3 4/9] mm: Export alloc_contig_range() / free_contig_range()

[David Hildenbrand]

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 <akpm@xxxxxxxxxxxxxxxxxxxx>
> > Cc: Michal Hocko <mhocko@xxxxxxxx>
> > Cc: Vlastimil Babka <vbabka@xxxxxxx>
> > Cc: Oscar Salvador <osalvador@xxxxxxx>
> > Cc: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
> > Cc: Mike Rapoport <rppt@xxxxxxxxxxxxx>
> > Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
> > Cc: Alexander Duyck <alexander.h.duyck@xxxxxxxxxxxxxxx>
> > Cc: Pavel Tatashin <pavel.tatashin@xxxxxxxxxxxxx>
> > Cc: Alexander Potapenko <glider@xxxxxxxxxx>
> > Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
>
> 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):

https://lkml.org/lkml/2019/9/19/486

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

Thanks!

--

Thanks,

David / dhildenb