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Subject: Re: [PATCH] mm/huge_memory: disable thp if thp page size is too large
To:     Hao Peng <flyingpenghao@gmail.com>, akpm@linux-foundation.org
Cc:     linux-mm@kvack.org, linux-kernel@vger.kernel.org
References: <CAPm50aLBfe7N_udxCkS3-PKbTmUvSpqu0sL7U6qHXZ=OWGQnTQ@mail.gmail.com>
From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat
Message-ID: <914a0b38-cb44-f4be-43aa-bc42bee1553b@redhat.com>
Date:   Thu, 30 Sep 2021 15:19:19 +0200
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On 30.09.21 04:14, Hao Peng wrote:
> From: Peng Hao <flyingpeng@tencent.com>

"disable thp if thp page size is too large" you disable thp if there is 
not sufficient memory installed in the system (and for that, you use THP 
size), something doesn't add up here.

> 
> After seting the page size to 64k on ARM64, the supported huge page

s/seting/setting/

Fortunately, most distributions already switched to 4k, because 512MB 
THP is pretty much useless, especially on any system that doesn't have 
memory in the range of hundreds of megabytes or terrabytes.

> size is 512M and 1TB. Therefore, if the thp is enabled, the size
> of the thp is 512M. In this case, min_free_kbytes will be too large.
> 
> On an arm64 server with 64G memory, the page size is 64k, with thp
> enabled.
> cat /proc/sys/vm/min_free_kbytes
> 3335104
> 
> Therefore, when judging whether to enable thp by default, consider
> the size of thp.
> 
> Signed-off-by: Peng Hao <flyingpeng@tencent.com>
> ---
>   mm/huge_memory.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 5e9ef0fc261e..03c7f571b3ae 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -437,7 +437,7 @@ static int __init hugepage_init(void)
>           * where the extra memory used could hurt more than TLB overhead
>           * is likely to save.  The admin can still enable it through /sys.
>           */
> -       if (totalram_pages() < (512 << (20 - PAGE_SHIFT))) {
> +       if (totalram_pages() < (512 << (HPAGE_PMD_SHIFT - PAGE_SHIFT))) {
>                  transparent_hugepage_flags = 0;
>                  return 0;
>          }
> --
> 2.27.0
> 


-- 
Thanks,

David / dhildenb