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

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Date:   Wed, 26 Oct 2022 14:44:04 +0200
From:   Gerald Schaefer <gerald.schaefer@...ux.ibm.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Shuah Khan <shuah@...nel.org>,
        Muchun Song <songmuchun@...edance.com>,
        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 Tue, 25 Oct 2022 17:34:52 +0200
David Hildenbrand <david@...hat.com> wrote:

> 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
> 

Hello David, nice, I guess that would be much better than adding new #ifdefs,
and also allow to check all available hugepage sizes per arch.