linux-kernel - Re: [RFC PATCH] mm/slab: Avoid build bug for calls to kmalloc with a large constant

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Message-ID: <66ecc29b-d46f-4eb5-a8b1-5570649f4b3a@oracle.com>
Date: Wed, 6 Nov 2024 09:20:23 -0600
From: Dave Kleikamp <dave.kleikamp@...cle.com>
To: Ryan Roberts <ryan.roberts@....com>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc: linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org
Subject: Re: [RFC PATCH] mm/slab: Avoid build bug for calls to kmalloc with a
 large constant

On 11/6/24 5:44AM, Ryan Roberts wrote:
> On 01/11/2024 20:16, Dave Kleikamp wrote:
>> When boot-time page size is enabled, the test against KMALLOC_MAX_CACHE_SIZE
>> is no longer optimized out with a constant size, so a build bug may
>> occur on a path that won't be reached.
>>
>> Found compiling drivers/net/ethernet/qlogic/qed/qed_sriov.c
>>
>> Signed-off-by: Dave Kleikamp <dave.kleikamp@...cle.com>
>> ---
>>
>> Ryan,
>>
>> Please consider incorporating this fix or something similar into your
>> mm patch in the boot-time pages size patches.
>>
>>   include/linux/slab.h | 3 ++-
>>   1 file changed, 2 insertions(+), 1 deletion(-)
>>
>> diff --git a/include/linux/slab.h b/include/linux/slab.h
>> index 9848296ca6ba..a4c7507ab8ec 100644
>> --- a/include/linux/slab.h
>> +++ b/include/linux/slab.h
>> @@ -685,7 +685,8 @@ static __always_inline unsigned int __kmalloc_index(size_t
>> size,
>>       if (size <= 1024 * 1024) return 20;
>>       if (size <=  2 * 1024 * 1024) return 21;
>>   
>> -    if (!IS_ENABLED(CONFIG_PROFILE_ALL_BRANCHES) && size_is_constant)
>> +    if (!IS_ENABLED(CONFIG_ARM64_BOOT_TIME_PAGE_SIZE) &&
> 
> Thanks for the patch! I think this may be better as:
> 
>         if (PAGE_SHIFT_MIN == PAGE_SHIFT_MAX &&
> 
> Since that is independent of the architecture. Your approach wouldn't work if
> another arch wanted to enable boot time page size, or if arm64 dropped the
> Kconfig because it decided only boot time page size will be supported in future.

Absolutely. I may be sending some more. I haven't gotten to JFS yet, but 
that one is my responsibility.

Thanks,
Shaggy

> 
> Thanks,
> Ryan
> 
>> +        !IS_ENABLED(CONFIG_PROFILE_ALL_BRANCHES) && size_is_constant)
>>           BUILD_BUG_ON_MSG(1, "unexpected size in kmalloc_index()");
>>       else
>>           BUG();
>