Re: [PATCH v2] mm/slab: reject unsupported kmalloc() sizes

[Vlastimil Babka (SUSE)]

On 8/31/26 04:23, Zi Yan wrote:
> kmalloc() is used to allocate physically contiguous memory for kernel
> allocations. For requests larger than KMALLOC_MAX_CACHE_SIZE, kmalloc()
> uses the page allocator and can only support up to KMALLOC_MAX_SIZE. For
> request sizes bigger than KMALLOC_MAX_SIZE, the page allocator can emit a
> WARN because kmalloc() allocates an order greater than MAX_PAGE_ORDER. To
> make the KMALLOC_MAX_SIZE limit more explicit instead of relying on the
> page allocator check, add a KMALLOC_MAX_SIZE check inside kmalloc() to emit
> a WARN and return NULL for requested sizes bigger than KMALLOC_MAX_SIZE.
> Like the WARN in the page allocator and kvmalloc(), allow __GFP_NOWARN to
> suppress the WARN.
> 
> Suggested-by: "Vlastimil Babka (SUSE)" <vbabka@xxxxxxxxxx>
> Signed-off-by: Zi Yan <ziy@xxxxxxxxxx>

Thanks, added to mm/slab.git slab/for-next

> ---
> Instead of passing requests greater than KMALLOC_MAX_SIZE to the page
> allocator, reject them early inside kmalloc().
> ---
> Changes in v2:
> 1. Rejected kmalloc() request sizes bigger than KMALLOC_MAX_SIZE by
>    emitting a WARN and returning NULL inside kmalloc(). The WARN can be
>    suppressed by __GFP_NOWARN. So the page allocator will no longer see
>    excessively large requests from kmalloc().
> - Link to v1: https://patch.msgid.link/20260817-limit_kmalloc_size-v1-1-5bef487701cc@xxxxxxxxxx
> ---
>  mm/slub.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/slub.c b/mm/slub.c
> index f9b56cb439e70..23e57a8fe55b4 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -5342,7 +5342,12 @@ static void *___kmalloc_large_node(size_t size, gfp_t flags, int node)
>  {
>  	struct page *page;
>  	void *ptr = NULL;
> -	unsigned int order = get_order(size);
> +	unsigned int order;
> +
> +	if (WARN_ON_ONCE_GFP(size > KMALLOC_MAX_SIZE, flags))
> +		return NULL;
> +
> +	order = get_order(size);
>  
>  	if (unlikely(flags & GFP_SLAB_BUG_MASK))
>  		flags = kmalloc_fix_flags(flags);
> 
> ---
> base-commit: cee9395acd8043be0644b25c34bfa86623f2b935
> change-id: 20260817-limit_kmalloc_size-3a4a2c73beac
> 
> Best regards,
> --  
> Yan, Zi
>