Re: [PATCH 5/5] hugetlb: add support for gigantic page allocation at runtime

[Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>]

On Tue, Apr 08, 2014 at 03:02:20PM -0400, Luiz Capitulino wrote:
> HugeTLB is limited to allocating hugepages whose size are less than
> MAX_ORDER order. This is so because HugeTLB allocates hugepages via
> the buddy allocator. Gigantic pages (that is, pages whose size is
> greater than MAX_ORDER order) have to be allocated at boottime.
> 
> However, boottime allocation has at least two serious problems. First,
> it doesn't support NUMA and second, gigantic pages allocated at
> boottime can't be freed.
> 
> This commit solves both issues by adding support for allocating gigantic
> pages during runtime. It works just like regular sized hugepages,
> meaning that the interface in sysfs is the same, it supports NUMA,
> and gigantic pages can be freed.
> 
> For example, on x86_64 gigantic pages are 1GB big. To allocate two 1G
> gigantic pages on node 1, one can do:
> 
>  # echo 2 > \
>    /sys/devices/system/node/node1/hugepages/hugepages-1048576kB/nr_hugepages
> 
> And to free them all:
> 
>  # echo 0 > \
>    /sys/devices/system/node/node1/hugepages/hugepages-1048576kB/nr_hugepages
> 
> The one problem with gigantic page allocation at runtime is that it
> can't be serviced by the buddy allocator. To overcome that problem, this
> commit scans all zones from a node looking for a large enough contiguous
> region. When one is found, it's allocated by using CMA, that is, we call
> alloc_contig_range() to do the actual allocation. For example, on x86_64
> we scan all zones looking for a 1GB contiguous region. When one is found,
> it's allocated by alloc_contig_range().
> 
> One expected issue with that approach is that such gigantic contiguous
> regions tend to vanish as runtime goes by. The best way to avoid this for
> now is to make gigantic page allocations very early during system boot, say
> from a init script. Other possible optimization include using compaction,
> which is supported by CMA but is not explicitly used by this commit.
> 
> It's also important to note the following:
> 
>  1. Gigantic pages allocated at boottime by the hugepages= command-line
>     option can be freed at runtime just fine
> 
>  2. This commit adds support for gigantic pages only to x86_64. The
>     reason is that I don't have access to nor experience with other archs.
>     The code is arch indepedent though, so it should be simple to add
>     support to different archs
> 
>  3. I didn't add support for hugepage overcommit, that is allocating
>     a gigantic page on demand when
>    /proc/sys/vm/nr_overcommit_hugepages > 0. The reason is that I don't
>    think it's reasonable to do the hard and long work required for
>    allocating a gigantic page at fault time. But it should be simple
>    to add this if wanted
> 
> Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>

Looks good to me, thanks.

Reviewed-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>

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