Date:   Fri, 3 Apr 2020 08:56:57 -0700
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     Roman Gushchin <guro@...com>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     Michal Hocko <mhocko@...nel.org>, linux-mm@...ck.org,
        kernel-team@...com, linux-kernel@...r.kernel.org,
        Rik van Riel <riel@...riel.com>,
        Andreas Schaufler <andreas.schaufler@....de>
Subject: Re: [PATCH v3] mm: hugetlb: optionally allocate gigantic hugepages
 using cma 65;5803;1c Commit 944d9fec8d7a ("hugetlb: add support for gigantic
 page allocation at runtime") has added the run-time allocation of gigantic
 pages. However it actually works only at early stages of the system loading,
 when the majority of memory is free. After some time the memory gets
 fragmented by non-movable pages, so the chances to find a contiguous 1 GB
 block are getting close to zero. Even dropping caches manually doesn't help a
 lot.

On 3/11/20 3:09 PM, Roman Gushchin wrote:
> At large scale rebooting servers in order to allocate gigantic hugepages
> is quite expensive and complex. At the same time keeping some constant
> percentage of memory in reserved hugepages even if the workload isn't
> using it is a big waste: not all workloads can benefit from using 1 GB
> pages.
> 
> The following solution can solve the problem:
> 1) On boot time a dedicated cma area* is reserved. The size is passed
>    as a kernel argument.
> 2) Run-time allocations of gigantic hugepages are performed using the
>    cma allocator and the dedicated cma area
> 
> In this case gigantic hugepages can be allocated successfully with a
> high probability, however the memory isn't completely wasted if nobody
> is using 1GB hugepages: it can be used for pagecache, anon memory,
> THPs, etc.
> 
> * On a multi-node machine a per-node cma area is allocated on each node.
>   Following gigantic hugetlb allocation are using the first available
>   numa node if the mask isn't specified by a user.
> 
> Usage:
> 1) configure the kernel to allocate a cma area for hugetlb allocations:
>    pass hugetlb_cma=10G as a kernel argument
> 
> 2) allocate hugetlb pages as usual, e.g.
>    echo 10 > /sys/kernel/mm/hugepages/hugepages-1048576kB/nr_hugepages
> 
> If the option isn't enabled or the allocation of the cma area failed,
> the current behavior of the system is preserved.
> 
> x86 and arm-64 are covered by this patch, other architectures can be
> trivially added later.
> 
> v3:
>   - added fallback to the existing allocation mechanism
>   - added min/max checks
>   - switched to MiB in debug output
>   - removed percentage option
>   - added arch-specific order argument to determine an alignment
>   - added arm support
>   - fixed the !CONFIG_HUGETLBFS build
> 
>   Thanks to Michal, Mike, Andreas and Rik for ideas and suggestions!
> 
> v2:
>   -fixed !CONFIG_CMA build, suggested by Andrew Morton
> 
> Signed-off-by: Roman Gushchin <guro@...com>

It is a bit difficult to keep track of all the followup patches.  One
small issue below.

> ---
>  .../admin-guide/kernel-parameters.txt         |   7 ++
>  arch/arm64/mm/init.c                          |   6 +
>  arch/x86/kernel/setup.c                       |   4 +
>  include/linux/hugetlb.h                       |   8 ++
>  mm/hugetlb.c                                  | 116 ++++++++++++++++++
>  5 files changed, 141 insertions(+)
> 
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index 0c9894247015..9eb0df40643d 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -1452,6 +1452,13 @@
>  	hpet_mmap=	[X86, HPET_MMAP] Allow userspace to mmap HPET
>  			registers.  Default set by CONFIG_HPET_MMAP_DEFAULT.
>  
> +	hugetlb_cma=	[x86-64] The size of a cma area used for allocation
> +			of gigantic hugepages.
> +			Format: nn[KMGTPE]
> +
> +			If enabled, boot-time allocation of gigantic hugepages
> +			is skipped.
> +
>  	hugepages=	[HW,X86-32,IA-64] HugeTLB pages to allocate at boot.
>  	hugepagesz=	[HW,IA-64,PPC,X86-64] The size of the HugeTLB pages.
>  			On x86-64 and powerpc, this option can be specified
> diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c
> index b65dffdfb201..e42727e3568e 100644
> --- a/arch/arm64/mm/init.c
> +++ b/arch/arm64/mm/init.c
> @@ -29,6 +29,7 @@
>  #include <linux/mm.h>
>  #include <linux/kexec.h>
>  #include <linux/crash_dump.h>
> +#include <linux/hugetlb.h>
>  
>  #include <asm/boot.h>
>  #include <asm/fixmap.h>
> @@ -457,6 +458,11 @@ void __init arm64_memblock_init(void)
>  	high_memory = __va(memblock_end_of_DRAM() - 1) + 1;
>  
>  	dma_contiguous_reserve(arm64_dma32_phys_limit);
> +
> +#ifdef CONFIG_ARM64_4K_PAGES
> +	hugetlb_cma_reserve(PUD_SHIFT - PAGE_SHIFT);
> +#endif
> +
>  }

The documentation is already 'out of date' as you added support for arm64.
Not a huge deal as documentation rarely keeps up with code, but we should
at least be correct here.

I have a patch series in progress which cleans up existing hugetlb command
line processing.
https://lore.kernel.org/linux-mm/20200401183819.20647-1-mike.kravetz@oracle.com/

No need to make any changes here, but assuming this support goes in first
I would make the following changes as part of my series:
- Don't list architectures in Documentation.  Just say support is arch
  dependent.
- Introduce some mechanism to print an error if hugetlb_cma is specified
  on the command line, but not supported by architecture.  IIUC, no message
  is printed today.  IMO, this only becomes important if the documentation
  does not list supported architectures.
 
Not insisting that documentation be updated to include arm64.
Acked-by: Mike Kravetz <mike.kravetz@...cle.com>
-- 
Mike Kravetz

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