Re: [PATCH] memtest: use {READ,WRITE}_ONCE in memory scanning

[Nathan Chancellor <nathan@kernel.org>]

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;
>  		if (start_phys_aligned == last_bad + incr) {
>  			last_bad += incr;
> -- 
> 2.39.2
>