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SubjectRe: [PATCH] kmalloc: Return NULL instead of link failure
On Tue, 20 Jan 2009 10:53:26 -0500
Jeff Mahoney <jeffm@suse.com> wrote:

> -----BEGIN PGP SIGNED MESSAGE-----
> Hash: SHA1
>
> The SLAB kmalloc with a constant value isn't consistent with the other
> implementations because it bails out with __you_cannot_kmalloc_that_much
> rather than returning NULL and properly allowing the caller to fall back
> to vmalloc or take other action. This doesn't happen with a non-constant
> value or with SLOB or SLUB.
>
> Starting with 2.6.28, I've been seeing build failures on s390x. This is
> due to init_section_page_cgroup trying to allocate 2.5MB when the max
> size for a kmalloc on s390x is 2MB.
>
> It's failing because the value is constant. The workarounds at the call
> size are ugly and the caller shouldn't have to change behavior depending
> on what the backend of the API is.
>
> So, this patch eliminates the link failure and returns NULL like the
> other implementations.
>

OK by me, is that's what the other sl[abcd...xyz]b.c implementations
do.

That __you_cannot_kmalloc_that_much() thing has frequently been a PITA
anyway - some gcc versions flub the constant_p() test and end up
referencing __you_cannot_kmalloc_that_much() when the callsite was
passing a variable `size' arg.

> - ---
> include/linux/slab_def.h | 10 ++--------
> 1 file changed, 2 insertions(+), 8 deletions(-)
>
> - --- a/include/linux/slab_def.h
> +++ b/include/linux/slab_def.h
> @@ -43,10 +43,7 @@ static inline void *kmalloc(size_t size,
> i++;
> #include <linux/kmalloc_sizes.h>
> #undef CACHE
> - - {
> - - extern void __you_cannot_kmalloc_that_much(void);
> - - __you_cannot_kmalloc_that_much();
> - - }
> + return NULL;
> found:
> #ifdef CONFIG_ZONE_DMA
> if (flags & GFP_DMA)
> @@ -77,10 +74,7 @@ static inline void *kmalloc_node(size_t
> i++;
> #include <linux/kmalloc_sizes.h>
> #undef CACHE
> - - {
> - - extern void __you_cannot_kmalloc_that_much(void);
> - - __you_cannot_kmalloc_that_much();
> - - }
> + return NULL;
> found:
> #ifdef CONFIG_ZONE_DMA
> if (flags & GFP_DMA)
>

Strange patch format, but it applied.

I'll punt this patch in the Pekka direction.

Do you think we should include it in 2.6.28.x?


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