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

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Message-ID: <69746c3a-72af-4c28-8f04-bcfae7a78107@arm.com>
Date: Tue, 26 Nov 2024 14:53:11 +0000
From: Ryan Roberts <ryan.roberts@....com>
To: Vlastimil Babka <vbabka@...e.cz>, Dave Kleikamp
 <dave.kleikamp@...cle.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 26/11/2024 12:36, Vlastimil Babka wrote:
> On 11/26/24 13:18, Ryan Roberts wrote:
>> On 14/11/2024 10:09, Vlastimil Babka wrote:
>>> On 11/1/24 21:16, 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.
>>>
>>> That's rather unfortunate, the __builtin_constant_p(size) part of
>>> kmalloc_noprof() really expects things to resolve at compile time and it
>>> would be better to keep it that way.
>>>
>>> I think it would be better if we based KMALLOC_MAX_CACHE_SIZE itself on
>>> PAGE_SHIFT_MAX and kept it constant, instead of introducing
>>> KMALLOC_SHIFT_HIGH_MAX only for some sanity checks.
>>>
>>> So if the kernel was built to support 4k to 64k, but booted as 4k, it would
>>> still create and use kmalloc caches up to 128k. SLUB should handle that fine
>>> (if not, please report it :)
>>
>> So when PAGE_SIZE_MAX=64K and PAGE_SIZE=4K, kmalloc will support up to 128K
>> whereas before it only supported up to 8K. I was trying to avoid that since I
>> assumed that would be costly in terms of extra memory allocated for those higher
>> order buckets that will never be used. But I have no idea how SLUB works in
>> practice. Perhaps memory for the cache is only lazily allocated so we won't see
>> an issue in practice?
> 
> Yes the e.g. 128k slabs themselves will be lazily allocated. There will be
> some overhead with the management structures (struct kmem_cache etc) but
> much smaller.
> To be completely honest, some extra overhead might come to be when the slabs
> are allocated ans later the user frees those allocations. kmalloc_large()
> wwould return them immediately, while a regular kmem_cache will keep one or
> more per cpu for reuse. But if that becomes a visible problem we can tune
> those caches to discard slabs more aggressively.

Sorry to keep pushing on this, now that I've actually looked at the code, I feel
I have a slightly better understanding:

void *kmalloc_noprof(size_t size, gfp_t flags)
{
	if (__builtin_constant_p(size) && size) {
		
		if (size > KMALLOC_MAX_CACHE_SIZE)
			return __kmalloc_large_noprof(size, flags); <<< (1)

		index = kmalloc_index(size);
		return __kmalloc_cache_noprof(...);   <<< (2)
	}
	return __kmalloc_noprof(size, flags);   <<< (3)
}

So if size and KMALLOC_MAX_CACHE_SIZE are constant, we end up with this
resolving either to a call to (1) or (2), decided at compile time. If
KMALLOC_MAX_CACHE_SIZE is not constant, (1), (2) and the runtime conditional
need to be kept in the function.

But intuatively, I would have guessed that given the choice between the overhead
of keeping that runtime conditional vs keeping per-cpu slab caches for extra
sizes between 16K and 128K, then the runtime conditional would be preferable. I
would guess that quite a bit of memory could get tied up in those caches?

Why is your preference the opposite? What am I not understanding?


> 
>> I'm happy to make this change if you're certain it's the right approach; please
>> confirm.
> 
> Yes it's much better option than breaking the build-time-constant part of
> kmalloc_noprof().
> 
>>>
>>> Maybe we could also stop adding + 1 to PAGE_SHIFT_MAX if it's >=64k, so the
>>> cache size is max 64k and not 128k but that should be probably evaluated
>>> separately from this series.
>>
>> I'm inferring from this that perhaps there is a memory cost with having the
>> higher orders defined but unused.
> 
> Yeah as per above, should not be too large and we could tune it down if
> necessary.
> 
>> Thanks,
>> Ryan
>>
>>>
>>> Vlastimil
>>>
>>>> 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) &&
>>>> +	    !IS_ENABLED(CONFIG_PROFILE_ALL_BRANCHES) && size_is_constant)
>>>>   		BUILD_BUG_ON_MSG(1, "unexpected size in kmalloc_index()");
>>>>   	else
>>>>   		BUG();
>>>
>>
>