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

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Message-ID: <20191016112051.GW317@dhcp22.suse.cz>
Date:   Wed, 16 Oct 2019 13:20:51 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     David Hildenbrand <david@...hat.com>
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 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.

> 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.

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

> ---
>  mm/page_alloc.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 3334a769eb91..d5d7944954b3 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -8469,6 +8469,7 @@ int alloc_contig_range(unsigned long start, unsigned long end,
>  				pfn_max_align_up(end), migratetype);
>  	return ret;
>  }
> +EXPORT_SYMBOL_GPL(alloc_contig_range);
>  #endif /* CONFIG_CONTIG_ALLOC */
>  
>  void free_contig_range(unsigned long pfn, unsigned int nr_pages)
> @@ -8483,6 +8484,7 @@ void free_contig_range(unsigned long pfn, unsigned int nr_pages)
>  	}
>  	WARN(count != 0, "%d pages are still in use!\n", count);
>  }
> +EXPORT_SYMBOL_GPL(free_contig_range);
>  
>  #ifdef CONFIG_MEMORY_HOTPLUG
>  /*
> -- 
> 2.21.0

-- 
Michal Hocko
SUSE Labs