Re: [RFC PATCH] mm/slab: Avoid build bug for calls to kmalloc with a large constant

Vlastimil Babka
2024-11-14T11:00:21Z

On 11/1/24 21:16, Dave Kleikamp wrote:
> When boot-time page size is enabled, the test against KMALLOC_MAX_CACHE_SIZE
> is no longer optimized out with a constant size, so a build bug may
> occur on a path that won't be reached.

That's rather unfortunate, the __builtin_constant_p(size) part of
kmalloc_noprof() really expects things to resolve at compile time and it
would be better to keep it that way.

I think it would be better if we based KMALLOC_MAX_CACHE_SIZE itself on
PAGE_SHIFT_MAX and kept it constant, instead of introducing
KMALLOC_SHIFT_HIGH_MAX only for some sanity checks.

So if the kernel was built to support 4k to 64k, but booted as 4k, it would
still create and use kmalloc caches up to 128k. SLUB should handle that fine
(if not, please report it :)

Maybe we could also stop adding + 1 to PAGE_SHIFT_MAX if it's >=64k, so the
cache size is max 64k and not 128k but that should be probably evaluated
separately from this series.

Vlastimil

> Found compiling drivers/net/ethernet/qlogic/qed/qed_sriov.c
>
> Signed-off-by: Dave Kleikamp <dave.kleikamp@oracle.com>
> ---
>
> Ryan,
>
> Please consider incorporating this fix or something similar into your
> mm patch in the boot-time pages size patches.
>
> include/linux/slab.h | 3 ++-
> 1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/include/linux/slab.h b/include/linux/slab.h
> index 9848296ca6ba..a4c7507ab8ec 100644
> --- a/include/linux/slab.h
> +++ b/include/linux/slab.h
> @@ -685,7 +685,8 @@ static __always_inline unsigned int __kmalloc_index(size_t size,
> if (size <= 1024 * 1024) return 20;
> if (size <= 2 * 1024 * 1024) return 21;
>
> - if (!IS_ENABLED(CONFIG_PROFILE_ALL_BRANCHES) && size_is_constant)
> + if (!IS_ENABLED(CONFIG_ARM64_BOOT_TIME_PAGE_SIZE) &&
> + !IS_ENABLED(CONFIG_PROFILE_ALL_BRANCHES) && size_is_constant)
> BUILD_BUG_ON_MSG(1, "unexpected size in kmalloc_index()");
> else
> BUG();