linux-kernel - Re: [PATCH v1 04/11] mm: Export alloc_contig_range() / free_contig

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Date:   Mon, 2 Mar 2020 15:17:38 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        virtio-dev@...ts.oasis-open.org,
        virtualization@...ts.linux-foundation.org, kvm@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Michael S . Tsirkin" <mst@...hat.com>,
        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 v1 04/11] mm: Export alloc_contig_range() /
 free_contig_range()

On 02.03.20 15:05, Michal Hocko wrote:
> On Mon 02-03-20 14:49:34, 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).
> 
> Same feedback as in pxm_to_node export. No objections to exporting the
> symbol but it would be better to squash this function into the patch
> which uses it. The changelog is highly virtio-mem specific anyway.
> Maybe it is just a dejavu but I feel I have already said that but I do
> not remember any details.

As I said back then, I am not a friend of squashing core changes into
driver changes (and AFAIK separating such is the common practice - well
I have never written a driver myself). I doubt it will make review
easier or faster (especially when it comes to patch #1).

I can squash #4 into #5, #6 into #7, #8 into #9 if it makes your review
easier.

-- 
Thanks,

David / dhildenb