linux-kernel - Re: [PATCH] selftests: vm: use 1 MB hugepage size for s390

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Date:   Tue, 25 Oct 2022 17:34:52 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Gerald Schaefer <gerald.schaefer@...ux.ibm.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Shuah Khan <shuah@...nel.org>,
        Muchun Song <songmuchun@...edance.com>
Cc:     linux-mm <linux-mm@...ck.org>, LKML <linux-kernel@...r.kernel.org>,
        linux-kselftest@...r.kernel.org,
        linux-s390 <linux-s390@...r.kernel.org>
Subject: Re: [PATCH] selftests: vm: use 1 MB hugepage size for s390

On 25.10.22 17:26, Gerald Schaefer wrote:
> hugepage-vmemmap test fails for s390 because it assumes a hugepagesize
> of 2 MB, while we have 1 MB on s390. This results in iterating over two
> hugepages. If they are consecutive in memory, check_page_flags() will
> stumble over the additional head page. Otherwise, it will stumble over
> non-huge pageflags, after crossing the first 1 MB hugepage.
> 
> Fix this by using 1 MB MAP_LENGTH for s390.
> 
> Signed-off-by: Gerald Schaefer <gerald.schaefer@...ux.ibm.com>
> ---
>   tools/testing/selftests/vm/hugepage-vmemmap.c | 7 +++++++
>   1 file changed, 7 insertions(+)
> 
> diff --git a/tools/testing/selftests/vm/hugepage-vmemmap.c b/tools/testing/selftests/vm/hugepage-vmemmap.c
> index 557bdbd4f87e..a4695f138cec 100644
> --- a/tools/testing/selftests/vm/hugepage-vmemmap.c
> +++ b/tools/testing/selftests/vm/hugepage-vmemmap.c
> @@ -11,7 +11,14 @@
>   #include <sys/mman.h>
>   #include <fcntl.h>
>   
> +/*
> + * 1 MB hugepage size for s390
> + */
> +#if defined(__s390x__)
> +#define MAP_LENGTH		(1UL * 1024 * 1024)
> +#else
>   #define MAP_LENGTH		(2UL * 1024 * 1024)
> +#endif

Why not detect it at runtime, so this works on any architecture (e.g., 
ppc64 with 16 MiB IIRC and arm64 with weird sizes)?

A patch that adds such detection code is currently on its way upstream:

https://lore.kernel.org/all/20220927110120.106906-5-david@redhat.com/T/#u

We could factor that out into vm_utils.c

-- 
Thanks,

David / dhildenb