Re: [PATCH 5.4 and earlier only] mm: Fix '.data.once' orphan section warning

[Hugh Dickins <hughd@google.com>]

On Mon, 28 Nov 2022, Nathan Chancellor wrote:

> Portions of upstream commit a4055888629b ("mm/memcg: warning on !memcg
> after readahead page charged") were backported as commit cfe575954ddd
> ("mm: add VM_WARN_ON_ONCE_PAGE() macro"). Unfortunately, the backport
> did not account for the lack of commit 33def8498fdd ("treewide: Convert
> macro and uses of __section(foo) to __section("foo")") in kernels prior
> to 5.10, resulting in the following orphan section warnings on PowerPC
> clang builds with CONFIG_DEBUG_VM=y:
> 
>   powerpc64le-linux-gnu-ld: warning: orphan section `".data.once"' from `mm/huge_memory.o' being placed in section `".data.once"'
>   powerpc64le-linux-gnu-ld: warning: orphan section `".data.once"' from `mm/huge_memory.o' being placed in section `".data.once"'
>   powerpc64le-linux-gnu-ld: warning: orphan section `".data.once"' from `mm/huge_memory.o' being placed in section `".data.once"'
> 
> This is a difference between how clang and gcc handle macro
> stringification, which was resolved for the kernel by not stringifying
> the argument to the __section() macro. Since that change was deemed not
> suitable for the stable kernels by commit 59f89518f510 ("once: fix
> section mismatch on clang builds"), do that same thing as that change
> and remove the quotes from the argument to __section().
> 
> Fixes: cfe575954ddd ("mm: add VM_WARN_ON_ONCE_PAGE() macro")
> Signed-off-by: Nathan Chancellor <nathan@kernel.org>

Yes indeed: thanks Nathan, sorry about that.

Acked-by: Hugh Dickins <hughd@google.com>

> ---
> 
> As far as I can tell, this should be applied to 5.4 and earlier. It
> should apply cleanly but let me know if not.

I think it should be good for 4.19 also, but I don't know what happens
or would happen in 4.14 and 4.9 trees, since those have no other example
of .data.once or ".data.once" (and I've lost what little I ever knew of
that linker script stuff).

Since we're not hearing complaints about those (or are you?), perhaps
those trees are not clang-ready in other ways, and for gcc it all works
out by itself: I'd be inclined to just leave them as is myself, if there
are no reports of breakage; but you may know better, and prefer to remove
the ' __section(".data.once")' from the 4.14 and 4.9 versions.

> 
>  include/linux/mmdebug.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/include/linux/mmdebug.h b/include/linux/mmdebug.h
> index 5d0767cb424a..4ed52879ce55 100644
> --- a/include/linux/mmdebug.h
> +++ b/include/linux/mmdebug.h
> @@ -38,7 +38,7 @@ void dump_mm(const struct mm_struct *mm);
>  		}							\
>  	} while (0)
>  #define VM_WARN_ON_ONCE_PAGE(cond, page)	({			\
> -	static bool __section(".data.once") __warned;			\
> +	static bool __section(.data.once) __warned;			\
>  	int __ret_warn_once = !!(cond);					\
>  									\
>  	if (unlikely(__ret_warn_once && !__warned)) {			\
> 
> base-commit: 4d2a309b5c28a2edc0900542d22fec3a5a22243b
> -- 
> 2.38.1