Re: [PATCH 4/4] hugetlb: add support for gigantic page allocation at runtime

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

On Wed, Apr 02, 2014 at 02:08:48PM -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 later:
> 
>  # 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
> series 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 time goes by. The best way to avoid this for
> now is to make gigantic page allocations very early during 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. My target systems are x86_64 machines, so I have only tested 1GB
>     pages allocation/release. I did try to make this arch indepedent
>     and expect it to work on other archs but didn't try it myself
> 
>  2. 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>

I agree to the basic idea. One question below ...

> ---
>  arch/x86/include/asm/hugetlb.h |  10 +++
>  mm/hugetlb.c                   | 177 ++++++++++++++++++++++++++++++++++++++---
>  2 files changed, 176 insertions(+), 11 deletions(-)
> 
> diff --git a/arch/x86/include/asm/hugetlb.h b/arch/x86/include/asm/hugetlb.h
> index a809121..2b262f7 100644
> --- a/arch/x86/include/asm/hugetlb.h
> +++ b/arch/x86/include/asm/hugetlb.h
> @@ -91,6 +91,16 @@ static inline void arch_release_hugepage(struct page *page)
>  {
>  }
>  
> +static inline int arch_prepare_gigantic_page(struct page *page)
> +{
> +	return 0;
> +}
> +
> +static inline void arch_release_gigantic_page(struct page *page)
> +{
> +}
> +
> +
>  static inline void arch_clear_hugepage_flags(struct page *page)
>  {
>  }

These are defined only on arch/x86, but called in generic code.
Does it cause build failure on other archs?

Thanks,
Naoya Horiguchi

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