Re: [RFC PATCH bpf-next v1 01/16] mm/vmalloc: Add get_vm_area_align()

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Medium] mm: get_vm_area_align() silently ignores align if VM_IOREMAP is passed
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commit 60a153047292d8f40c9a75614c11c4a932d74c1f
Author: Kumar Kartikeya Dwivedi <memxor@gmail.com>

mm/vmalloc: Add get_vm_area_align()

This commit introduces get_vm_area_align(), a new variant of get_vm_area()
that allows callers to specify a custom alignment for the start of the
virtual memory area. This avoids wasting virtual address space when naturally
aligned regions are required, such as for BPF arena maps.

> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> index bea9f76ed7e74..b70e4e27ec362 100644
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c

[ ... ]

> @@ -3301,6 +3301,25 @@ struct vm_struct *get_vm_area_caller(unsigned long size, unsigned long flags,
>  				  NUMA_NO_NODE, GFP_KERNEL, caller);
>  }
>  
> +/**
> + * get_vm_area_align - reserve a contiguous kernel virtual area at an aligned start
> + * @size:	 size of the area
> + * @align:	 alignment of the start of the area, a power of two
> + * @flags:	 %VM_IOREMAP for I/O mappings or VM_ALLOC

[Severity: Medium]
Does this API work as intended if a caller passes VM_IOREMAP?

The documentation explicitly invites the caller to pass VM_IOREMAP, but
__get_vm_area_node() unconditionally overwrites the requested alignment
when this flag is set:

mm/vmalloc.c:__get_vm_area_node() {
    ...
    if (flags & VM_IOREMAP)
        align = 1ul << clamp_t(int, get_count_order_long(size),
                               PAGE_SHIFT, IOREMAP_MAX_ORDER);
    ...
}

Will this establish a broken contract where the caller's explicitly
requested alignment is silently ignored and overridden based on the
size of the area?

> + *
> + * Like get_vm_area(), with the start of the area aligned to @align.
> + *
> + * Return: the area descriptor on success or %NULL on failure.
> + */
> +struct vm_struct *get_vm_area_align(unsigned long size, unsigned long align,
> +				    unsigned long flags)
> +{
> +	return __get_vm_area_node(size, align, PAGE_SHIFT, flags,
> +				  VMALLOC_START, VMALLOC_END,
> +				  NUMA_NO_NODE, GFP_KERNEL,
> +				  __builtin_return_address(0));
> +}
> +

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260926233503.3114147-1-memxor@gmail.com?part=1