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

[Vlastimil Babka (SUSE)]

On 8/27/26 17:51, Zi Yan wrote:
> On Thu Aug 27, 2026 at 3:47 AM EDT, Vlastimil Babka (SUSE) wrote:
>> On 8/26/26 11:52 PM, Zi Yan wrote:
>>> On Wed Aug 26, 2026 at 5:47 PM EDT, Harry Yoo wrote:
>>>> On Wed, Aug 26, 2026 at 11:28:12AM +0000, Vlastimil Babka (SUSE) wrote:
>>>>> On 8/17/26 22:40, 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. Systems
>>>>>> with panic_on_warn=1 crash because of this WARN. Fix it by rejecting any
>>>>>> kmalloc size bigger than KMALLOC_MAX_SIZE.
>>>>>>
>>>>>> Fixes: aadb4bc4a1f9 ("SLUB: direct pass through of page size or higher kmalloc requests")
>>>>>> Reported-by: syzbot+805630f1453e490427fa@xxxxxxxxxxxxxxxxxxxxxxxxx
>>>>>> Closes: https://lore.kernel.org/all/6a820ebc.9ebadd4d.20b15e.001b.GAE@xxxxxxxxxx/
>>>>>> Tested-by: syzbot+805630f1453e490427fa@xxxxxxxxxxxxxxxxxxxxxxxxx
>>>>>> Signed-off-by: Zi Yan <ziy@xxxxxxxxxx>
>>>>>> Cc: stable@xxxxxxxxxxxxxxx
>>>>>> ---
>>>>>> It fixes a page allocator warning (order > MAX_PAGE_ORDER) when gadgetfs
>>>>>> requests excessively large memory from kmalloc. Instead of adding
>>>>>> __GFP_NOWARN to suppress the warning, as was done for usbfs[1], change
>>>>>> kmalloc to return NULL without a warning for this specific issue.
>>>>>>
>>>>>> [1] commit 4f2629ea67e72 ("USB: usbfs: Don't WARN about excessively large memory allocations")
>>>>>
>>>>> So I checked and for kvmalloc() we have in __kvmalloc_node_noprof()
>>>>>
>>>>>         /* Don't even allow crazy sizes */
>>>>>         if (unlikely(size > INT_MAX)) {
>>>>>                 WARN_ON_ONCE(!(flags & __GFP_NOWARN));
>>>>>                 return NULL;
>>>>>         }
>>>>>
>>>>> This comes from Linus in commit 7661809d493b4. I'd do the same thing here
>>>>> then.
>>>>
>>>> But the purpose of this patch is to avoid the warning in the page
>>>> allocator. Should we fix this in the caller (gadgetfs) then? 
>>> 
>>> It is fixed by: https://lore.kernel.org/all/20260820223719.A4A3C1F000E9@xxxxxxxxxxxxxxx/
>>> 
>>> Please disregard this patch, but we can keep the discussion going.
>>
>> I still think this patch has some value if done as proposed above. Yes
>> in practice it will just replace the page allocator's warning with a
>> different warning, but IMHO it's "nicer" if kmalloc() sanitizes its own
> 
> And SLAB maintainers will be Cc'd. :)

For some people it doesn't matter if it's SLAB or PAGE ALLOCATOR :D

>> requests to the page allocator, using the KMALLOC_MAX_SIZE value.
> 
> Like this? Or the exact pattern as kvmalloc() is preferred?

LGTM. Should return NULL even with __GFP_NOWARN or if the warning has fired
and won't again, and AFAICS this does.

The kvmalloc() pattern predates WARN_ON_ONCE_GFP addition, I think.

> diff --git a/mm/slub.c b/mm/slub.c
> index 0337e60db5ace..b562f2a6fbbee 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -5263,7 +5263,12 @@ 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);
> 
>