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Message-ID: <20250807131611.430a097a@pumpkin>
Date: Thu, 7 Aug 2025 13:16:11 +0100
From: David Laight <david.laight.linux@...il.com>
To: Zi Yan <ziy@...dia.com>
Cc: wang lian <lianux.mm@...il.com>, akpm@...ux-foundation.org,
broonie@...nel.org, david@...hat.com, lorenzo.stoakes@...cle.com,
sj@...nel.org, linux-mm@...ck.org, linux-kernel@...r.kernel.org,
brauner@...nel.org, gkwang@...x-info.com, jannh@...gle.com,
Liam.Howlett@...cle.com, ludovico.zy.wu@...il.com, p1ucky0923@...il.com,
richard.weiyang@...il.com, ryncsn@...il.com, shuah@...nel.org,
vbabka@...e.cz, zijing.zhang@...ton.me
Subject: Re: [PATCH 1/2] selftests/mm: reuse FORCE_READ to replace "asm
volatile("" : "+r" (XXX));"
On Tue, 05 Aug 2025 10:26:17 -0400
Zi Yan <ziy@...dia.com> wrote:
> On 17 Jul 2025, at 9:18, wang lian wrote:
>
> > Several mm selftests use the `asm volatile("" : "+r" (variable));`
> > construct to force a read of a variable, preventing the compiler from
> > optimizing away the memory access. This idiom is cryptic and duplicated
> > across multiple test files.
> >
> > Following a suggestion from David[1], this patch refactors this
> > common pattern into a FORCE_READ() macro
> >
> > [1] https://lore.kernel.org/lkml/4a3e0759-caa1-4cfa-bc3f-402593f1eee3@redhat.com/
> >
> > Signed-off-by: wang lian <lianux.mm@...il.com>
> > Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
> > ---
> > tools/testing/selftests/mm/cow.c | 30 +++++++++----------
> > tools/testing/selftests/mm/guard-regions.c | 7 -----
> > tools/testing/selftests/mm/hugetlb-madvise.c | 5 +---
> > tools/testing/selftests/mm/migration.c | 13 ++++----
> > tools/testing/selftests/mm/pagemap_ioctl.c | 4 +--
> > .../selftests/mm/split_huge_page_test.c | 4 +--
> > tools/testing/selftests/mm/vm_util.h | 7 +++++
> > 7 files changed, 31 insertions(+), 39 deletions(-)
> >
>
> <snip>
>
> > diff --git a/tools/testing/selftests/mm/split_huge_page_test.c b/tools/testing/selftests/mm/split_huge_page_test.c
> > index f0d9c035641d..05de1fc0005b 100644
> > --- a/tools/testing/selftests/mm/split_huge_page_test.c
> > +++ b/tools/testing/selftests/mm/split_huge_page_test.c
> > @@ -399,7 +399,6 @@ int create_pagecache_thp_and_fd(const char *testfile, size_t fd_size, int *fd,
> > char **addr)
> > {
> > size_t i;
> > - int dummy = 0;
> > unsigned char buf[1024];
> >
> > srand(time(NULL));
> > @@ -441,8 +440,7 @@ int create_pagecache_thp_and_fd(const char *testfile, size_t fd_size, int *fd,
> > madvise(*addr, fd_size, MADV_HUGEPAGE);
> >
> > for (size_t i = 0; i < fd_size; i++)
> > - dummy += *(*addr + i);
> > - asm volatile("" : "+r" (dummy));
> > + FORCE_READ((*addr + i));
>
> I encountered a segfault when running the test on x86_64.
> i is 4194297 and fd_size is 4194304.
> It seems that FORCE_READ() is reading (*addr + i) in 8 byte size
> and i is only 7 bytes away from the end of the memory address.
> This led to segfault.
>
> (*(volatile char*)(*addr + i)); works fine.
>
> Both gcc-12 and gcc-14 have the issue.
The definition of FORCE_READ in 6.16 is:
#define FORCE_READ(x) (*(volatile typeof(x) *)x)
this is clearly bogus.
'x' is a pointer - follow it through.
Possibly:
#define FORCE_READ(x) (*(volatile typeof(*(x)) *)(x))
is better,
But why not use READ_ONCE(*addr[i]) ?
David
>
> >
> > if (!check_huge_file(*addr, fd_size / pmd_pagesize, pmd_pagesize)) {
> > ksft_print_msg("No large pagecache folio generated, please provide a filesystem supporting large folio\n");
> > diff --git a/tools/testing/selftests/mm/vm_util.h b/tools/testing/selftests/mm/vm_util.h
> > index 2b154c287591..c20298ae98ea 100644
> > --- a/tools/testing/selftests/mm/vm_util.h
> > +++ b/tools/testing/selftests/mm/vm_util.h
> > @@ -18,6 +18,13 @@
> > #define PM_SWAP BIT_ULL(62)
> > #define PM_PRESENT BIT_ULL(63)
> >
> > +/*
> > + * Ignore the checkpatch warning, we must read from x but don't want to do
> > + * anything with it in order to trigger a read page fault. We therefore must use
> > + * volatile to stop the compiler from optimising this away.
> > + */
> > +#define FORCE_READ(x) (*(volatile typeof(x) *)x)
> > +
>
> Also, look at FORCE_READ again, it converts x to a pointer to x and
> deferences x as a point. It does not seem right to me.
>
> Best Regards,
> Yan, Zi
>
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