linux-kernel - Re: [PATCH RFC v3 4/9] mm: Export alloc_contig_range() / free_contig

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Date:   Wed, 16 Oct 2019 14:31:14 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        virtualization@...ts.linux-foundation.org,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Oscar Salvador <osalvador@...e.de>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Alexander Duyck <alexander.h.duyck@...ux.intel.com>,
        Pavel Tatashin <pavel.tatashin@...rosoft.com>,
        Alexander Potapenko <glider@...gle.com>
Subject: Re: [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 <akpm@...ux-foundation.org>
>> Cc: Michal Hocko <mhocko@...e.com>
>> Cc: Vlastimil Babka <vbabka@...e.cz>
>> Cc: Oscar Salvador <osalvador@...e.de>
>> Cc: Mel Gorman <mgorman@...hsingularity.net>
>> Cc: Mike Rapoport <rppt@...ux.ibm.com>
>> Cc: Dan Williams <dan.j.williams@...el.com>
>> Cc: Alexander Duyck <alexander.h.duyck@...ux.intel.com>
>> Cc: Pavel Tatashin <pavel.tatashin@...rosoft.com>
>> Cc: Alexander Potapenko <glider@...gle.com>
>> Signed-off-by: David Hildenbrand <david@...hat.com>
> 
> 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@...e.com> # to export contig range allocator API

Thanks!

-- 

Thanks,

David / dhildenb