Re: [PATCH 5/5] mm/vmalloc.c: return explicit error value in alloc_vmap_area()

[Shivank Garg]

On 4/15/2025 6:31 PM, Baoquan He wrote:
> On 04/15/25 at 12:52pm, Shivank Garg wrote:
>> On 4/15/2025 8:09 AM, Baoquan He wrote:
>>> In codes of alloc_vmap_area(), it returns the upper bound 'vend' to
>>> indicate if the allocation is successful or failed. That is not very clear.
>>>
>>> Here change to return explicit error values and check them to judge if
>>> allocation is successful.
>>>
>>> IS_ERR_VALUE already uses unlikely() internally
>>>
>>> Signed-off-by: Baoquan He <bhe@xxxxxxxxxx>
>>> ---
>>>  mm/vmalloc.c | 34 +++++++++++++++++-----------------
>>>  1 file changed, 17 insertions(+), 17 deletions(-)
>>>
>>> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
>>> index 3f38a232663b..5b21cd09b2b4 100644
>>> --- a/mm/vmalloc.c
>>> +++ b/mm/vmalloc.c
>>> @@ -1715,7 +1715,7 @@ va_clip(struct rb_root *root, struct list_head *head,
>>>  			 */
>>>  			lva = kmem_cache_alloc(vmap_area_cachep, GFP_NOWAIT);
>>>  			if (!lva)
>>> -				return -1;
>>> +				return -ENOMEM;
>>>  		}
>>>  
>>>  		/*
>>> @@ -1729,7 +1729,7 @@ va_clip(struct rb_root *root, struct list_head *head,
>>>  		 */
>>>  		va->va_start = nva_start_addr + size;
>>>  	} else {
>>> -		return -1;
>>> +		return -EINVAL;
>>>  	}
> 
> Thanks for reviewing.
> 
>>
>> Braces around return -EINVAL seem unnecessary.
>> They can be dropped.
> 
> This complys with the codeing style required in 3) Placing Braces and
> Spaces of Documentation/process/coding-style.rst because other branches
> are multiple statements.
> 
>>
>>>  
>>>  	if (type != FL_FIT_TYPE) {
>>> @@ -1758,19 +1758,19 @@ va_alloc(struct vmap_area *va,
>>>  
>>>  	/* Check the "vend" restriction. */
>>>  	if (nva_start_addr + size > vend)
>>> -		return vend;
>>> +		return -ERANGE;
>>>  
>>>  	/* Update the free vmap_area. */
>>>  	ret = va_clip(root, head, va, nva_start_addr, size);
>>> -	if (WARN_ON_ONCE(ret))
>>> -		return vend;
>>> +	if (ret)
>>> +		return ret;
>>
>> Is it safe to remove the warning, or was it critical for debugging?
> 
> This comes from a reported concern because va_clip() could be failed by 
> NOTHING_FIT or kmem_cache_alloc failure. The warning here could cause
> confusion misleading people to think vmap area management is failed.
> 
>>
>>>  
>>>  	return nva_start_addr;
>>>  }
>>>  
>>>  /*
>>>   * Returns a start address of the newly allocated area, if success.
>>> - * Otherwise a vend is returned that indicates failure.
>>> + * Otherwise an error value is returned that indicates failure.
>>>   */
>>>  static __always_inline unsigned long
>>>  __alloc_vmap_area(struct rb_root *root, struct list_head *head,
>>> @@ -1795,14 +1795,13 @@ __alloc_vmap_area(struct rb_root *root, struct list_head *head,
>>>  
>>>  	va = find_vmap_lowest_match(root, size, align, vstart, adjust_search_size);
>>>  	if (unlikely(!va))
>>> -		return vend;
>>> +		return -ENOENT;
>>>  
>>>  	nva_start_addr = va_alloc(va, root, head, size, align, vstart, vend);
>>> -	if (nva_start_addr == vend)
>>> -		return vend;
>>>  
>>>  #if DEBUG_AUGMENT_LOWEST_MATCH_CHECK
>>> -	find_vmap_lowest_match_check(root, head, size, align);
>>> +	if (!IS_ERR_VALUE(nva_start_addr))
>>> +		find_vmap_lowest_match_check(root, head, size, align);
>>>  #endif
>>>  
>>>  	return nva_start_addr;
>>> @@ -1932,7 +1931,7 @@ node_alloc(unsigned long size, unsigned long align,
>>>  	struct vmap_area *va;
>>>  
>>>  	*vn_id = 0;
>>> -	*addr = vend;
>>> +	*addr = -EINVAL;
>>>  
>>>  	/*
>>>  	 * Fallback to a global heap if not vmalloc or there
>>> @@ -2012,20 +2011,20 @@ static struct vmap_area *alloc_vmap_area(unsigned long size,
>>>  	}
>>>  
>>>  retry:
>>> -	if (addr == vend) {
>>> +	if (IS_ERR_VALUE(addr)) {
>>>  		preload_this_cpu_lock(&free_vmap_area_lock, gfp_mask, node);
>>>  		addr = __alloc_vmap_area(&free_vmap_area_root, &free_vmap_area_list,
>>>  			size, align, vstart, vend);
>>>  		spin_unlock(&free_vmap_area_lock);
>>>  	}
>>>  
>>> -	trace_alloc_vmap_area(addr, size, align, vstart, vend, addr == vend);
>>> +	trace_alloc_vmap_area(addr, size, align, vstart, vend, IS_ERR_VALUE(addr));
>>>  
>>>  	/*
>>> -	 * If an allocation fails, the "vend" address is
>>> +	 * If an allocation fails, the error value is
>>>  	 * returned. Therefore trigger the overflow path.
>>>  	 */
>>> -	if (unlikely(addr == vend))
>>> +	if (IS_ERR_VALUE(addr))
>>>  		goto overflow;
>>>  
>>>  	va->va_start = addr;
>>> @@ -4753,9 +4752,10 @@ struct vm_struct **pcpu_get_vm_areas(const unsigned long *offsets,
>>>  
>>>  		ret = va_clip(&free_vmap_area_root,
>>>  			&free_vmap_area_list, va, start, size);
>>> -		if (WARN_ON_ONCE(unlikely(ret)))
>>> -			/* It is a BUG(), but trigger recovery instead. */
>>> +		if ((unlikely(ret))) {
>> 		    ^^		   ^^
>> The extra parentheses are redundant and can be removed for clarity.
> 
> You are right, I will remove it. Thanks.
> 

Please feel free to add following in next version.

Reviewed-by: Shivank Garg <shivankg@xxxxxxx>
Tested-by: Shivank Garg <shivankg@xxxxxxx>

Thanks,
Shivank

>>
>>> +			WARN_ONCE(1, "%s error: errno (%d)\n", __func__, ret);
>>>  			goto recovery;
>>> +		}
>>>  
>>>  		/* Allocated area. */
>>>  		va = vas[area];
>>
>