memtest failed to find bad memory when compiled with clang. So use {WRITE,READ}_ONCE to access memory to avoid compiler over optimization.
Cc: Stable@vger.kernel.org Signed-off-by: Qiang Zhang qiang4.zhang@intel.com --- mm/memtest.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/mm/memtest.c b/mm/memtest.c index 32f3e9dda837..c2c609c39119 100644 --- a/mm/memtest.c +++ b/mm/memtest.c @@ -51,10 +51,10 @@ static void __init memtest(u64 pattern, phys_addr_t start_phys, phys_addr_t size last_bad = 0;
for (p = start; p < end; p++) - *p = pattern; + WRITE_ONCE(*p, pattern);
for (p = start; p < end; p++, start_phys_aligned += incr) { - if (*p == pattern) + if (READ_ONCE(*p) == pattern) continue; if (start_phys_aligned == last_bad + incr) { last_bad += incr;
On Tue, Mar 12, 2024 at 04:04:23PM +0800, Qiang Zhang wrote:
memtest failed to find bad memory when compiled with clang. So use {WRITE,READ}_ONCE to access memory to avoid compiler over optimization.
This commit message is severely lacking in details in my opinion, especially for a patch marked for stable. Did a kernel or LLVM change cause this (i.e., has this always been an issue or is it a recent regression)? What is the transformation that LLVM does to break the test and why is using READ_ONCE() or WRITE_ONCE() sufficient to resolve it?
Cc: Stable@vger.kernel.org Signed-off-by: Qiang Zhang qiang4.zhang@intel.com
mm/memtest.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/mm/memtest.c b/mm/memtest.c index 32f3e9dda837..c2c609c39119 100644 --- a/mm/memtest.c +++ b/mm/memtest.c @@ -51,10 +51,10 @@ static void __init memtest(u64 pattern, phys_addr_t start_phys, phys_addr_t size last_bad = 0; for (p = start; p < end; p++)
*p = pattern;
WRITE_ONCE(*p, pattern);for (p = start; p < end; p++, start_phys_aligned += incr) {
if (*p == pattern)
if (READ_ONCE(*p) == pattern) continue;-- 2.39.2
Hi, Nathan
Sorry for the incomplete commit message.
I have tried to compile with gcc and clang-{11,13,14} on Debian 12. On my test environment, hypervisor emulates a range of bad memory where writes are ignored and reads always returns all ones. Memtest compiled with all clang-{11,13,14} can't find the bad memory without this patch. But gcc works fine. So it seems not a regression in clang. I don't have expertise in compilers. But I think {READ,WRITE}_ONCE can force the compiler to treat the iterating pointer as volatile.
Welcome more comments !
BR Qiang
-----Original Message----- From: Nathan Chancellor nathan@kernel.org Sent: Thursday, March 14, 2024 1:22 AM To: Zhang, Qiang4 qiang4.zhang@intel.com Cc: Andrew Morton akpm@linux-foundation.org; Nick Desaulniers ndesaulniers@google.com; Bill Wendling morbo@google.com; Justin Stitt justinstitt@google.com; linux-mm@kvack.org; linux-kernel@vger.kernel.org; llvm@lists.linux.dev; Stable@vger.kernel.org Subject: Re: [PATCH] memtest: use {READ,WRITE}_ONCE in memory scanning
On Tue, Mar 12, 2024 at 04:04:23PM +0800, Qiang Zhang wrote:
memtest failed to find bad memory when compiled with clang. So use {WRITE,READ}_ONCE to access memory to avoid compiler over optimization.
This commit message is severely lacking in details in my opinion, especially for a patch marked for stable. Did a kernel or LLVM change cause this (i.e., has this always been an issue or is it a recent regression)? What is the transformation that LLVM does to break the test and why is using READ_ONCE() or WRITE_ONCE() sufficient to resolve it?
Cc: Stable@vger.kernel.org Signed-off-by: Qiang Zhang qiang4.zhang@intel.com
mm/memtest.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/mm/memtest.c b/mm/memtest.c index 32f3e9dda837..c2c609c39119 100644 --- a/mm/memtest.c +++ b/mm/memtest.c @@ -51,10 +51,10 @@ static void __init memtest(u64 pattern, phys_addr_t start_phys, phys_addr_t size last_bad = 0; for (p = start; p < end; p++)
*p = pattern;
WRITE_ONCE(*p, pattern);for (p = start; p < end; p++, start_phys_aligned += incr) {
if (*p == pattern)
if (READ_ONCE(*p) == pattern) continue;-- 2.39.2
linux-stable-mirror@lists.linaro.org
-
Nathan Chancellor -
Qiang Zhang -
Zhang, Qiang4