Re: [PATCH] mm/slab: call __kmalloc_noprof() when size isn't builtin constant in kmalloc_array_noprof()

[Vlastimil Babka <vbabka@suse.cz>]

On 3/2/25 14:30, Harry Yoo wrote:
> On Sat, Mar 01, 2025 at 04:37:20PM +0800, Ye Bin wrote:
>> From: Ye Bin <yebin10@huawei.com>
>> 
>> If 'n' or 'size' isn't builtin constant just call __kmalloc_noprof()
>> in kmalloc_array_noprof().
>> 
>> Fixes: 7bd230a26648 ("mm/slab: enable slab allocation tagging for kmalloc and friends")
>> Signed-off-by: Ye Bin <yebin10@huawei.com>
>> ---
>>  include/linux/slab.h | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>> 
>> diff --git a/include/linux/slab.h b/include/linux/slab.h
>> index 3b03b31831a9..0edb8363fc4f 100644
>> --- a/include/linux/slab.h
>> +++ b/include/linux/slab.h
>> @@ -947,7 +947,7 @@ static inline __alloc_size(1, 2) void *kmalloc_array_noprof(size_t n, size_t siz
>>  		return NULL;
>>  	if (__builtin_constant_p(n) && __builtin_constant_p(size))
>>  		return kmalloc_noprof(bytes, flags);
>> -	return kmalloc_noprof(bytes, flags);
>> +	return __kmalloc_noprof(bytes, flags);
> 
> Can we simply call kmalloc_noprof(bytes, flags) once instead?
> 
> The compiler should know 'bytes' is a constant if both 'n' and 'size' are
> constants.

Right, applied like this, hope it's ok?


commit a6553ee4b00ba4b1466a81fb00fa4679b7ce5339 (HEAD -> slab/for-6.15/fixes-cleanups)
Author: Ye Bin <yebin10@huawei.com>
Date:   Sat Mar 1 16:37:20 2025 +0800

    mm/slab: call kmalloc_noprof() unconditionally in kmalloc_array_noprof()
    
    If 'n' or 'size' isn't builtin constant, we used to call __kmalloc()
    before commit 7bd230a26648 ("mm/slab: enable slab allocation tagging for
    kmalloc and friends"), which inadvertedly changed both paths to
    kmalloc_noprof().
    
    As Harry Yoo points out we can just call kmalloc_noprof()
    unconditionally. If the compiler knows n and size are constants it
    doesn't guarantee that bytes will be also seen as constant, and that is
    the important test in kmalloc_noprof() here, so we can just defer to it
    always.
    
    [ vbabka@suse.cz: change as Harry suggested and adjust commit log ]
    
    Fixes: 7bd230a26648 ("mm/slab: enable slab allocation tagging for kmalloc and friends")
    Signed-off-by: Ye Bin <yebin10@huawei.com>
    Signed-off-by: Vlastimil Babka <vbabka@suse.cz>

diff --git a/include/linux/slab.h b/include/linux/slab.h
index 09eedaecf120..ab05a143d09a 100644
--- a/include/linux/slab.h
+++ b/include/linux/slab.h
@@ -941,8 +941,6 @@ static inline __alloc_size(1, 2) void *kmalloc_array_noprof(size_t n, size_t siz
 
        if (unlikely(check_mul_overflow(n, size, &bytes)))
                return NULL;
-       if (__builtin_constant_p(n) && __builtin_constant_p(size))
-               return kmalloc_noprof(bytes, flags);
        return kmalloc_noprof(bytes, flags);
 }
 #define kmalloc_array(...)                     alloc_hooks(kmalloc_array_noprof(__VA_ARGS__))