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

[Zhang, Qiang4]

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@xxxxxxxxxx> 
Sent: Thursday, March 14, 2024 1:22 AM
To: Zhang, Qiang4 <qiang4.zhang@xxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>; Nick Desaulniers <ndesaulniers@xxxxxxxxxx>; Bill Wendling <morbo@xxxxxxxxxx>; Justin Stitt <justinstitt@xxxxxxxxxx>; linux-mm@xxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; llvm@xxxxxxxxxxxxxxx; Stable@xxxxxxxxxxxxxxx
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@xxxxxxxxxxxxxxx>
> Signed-off-by: Qiang Zhang <qiang4.zhang@xxxxxxxxx>
> ---
>  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
>