linux-kernel - Re: [PATCH v3 00/15] common kmalloc v3

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Date:   Mon, 15 Aug 2022 14:59:55 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Hyeonggon Yoo <42.hyeyoo@...il.com>
Cc:     Christoph Lameter <cl@...ux.com>,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Joe Perches <joe@...ches.com>,
        Vasily Averin <vasily.averin@...ux.dev>,
        Matthew WilCox <willy@...radead.org>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v3 00/15] common kmalloc v3

On 8/14/22 12:06, Hyeonggon Yoo wrote:
> On Fri, Jul 29, 2022 at 05:08:31PM +0200, Vlastimil Babka wrote:
>> On 7/12/22 15:39, Hyeonggon Yoo wrote:
>>> This is v3 of common kmalloc series.
>>>
>>> This series generalize most of kmalloc functions and move its
>>> implementation into mm/slab_common.c.
>>>
>>> I believe this series give better maintainability of code for SLAB and SLUB.
>>> Please consider applying.
>>>
>>> This series is based on slab/for-next and also available at
>>> https://github.com/hygoni/linux/tree/slab-common-v3r0
>>>
>>> Any feedbacks/reviews will be appreciated.
>>
>> Hi, thanks for all your efforts. It's shaping up nicely so I think the next
>> version will be ready to be added to -next after the 5.20 merge window.
>> As I've finished the individual reviews, I'm looking at the result and see a
>> bit more potential for cleanups, which could be perhaps incorporated to
>> existing patches, or additionally:
> 
> Thank you for reviews and I too would like to add it to -next soon!
> 
>>
>> - kmalloc_large_node_notrace() has only one caller, can be removed and the
>> caller can call __kmalloc_large_node_notrace() directly, especially if it's
>> not __always_inline as I've IIRC suggested.
> 
> Will adjust in next version.
> 
>> - kmem_cache_alloc_trace() and kmem_cache_alloc_node_trace() are weird ones,
>> they are in fact for kmalloc despite the name.
> 
> Yeah, I'm the one that would like to rename it to kmalloc_trace() and
> kmalloc_node_trace().
> 
>> They depend on
>> CONFIG_TRACING, yet if you look carefully, the !CONFIG_TRACING variant also
>> goes through a trace_* function. The actual difference seems that
>> slab_alloc() thus kasan_alloc() and kfence_alloc() don't get the orig_size
>> that way, which is dubious. It might be worth trying to unify this as well?
>> E.g.
>>   - use only the CONFIG_TRACING variant, discard the other
> 
> Sounds okay.
> 
>>   - declare it in mm/slab.h, this is not for general usage
> 
> We can't completely remove it because its needed in include/linux/slab.h
> for inlined kmalloc.

Ah, ok.

>>   - single implementation in mm/slab_common.c that calls
>>     __kmem_cache_alloc_node() from SLAB/SLUB and does the trace
> 
> While I love the idea of single implementation in mm/slab_common.c,
> making use of __kmem_cache_alloc_node() and __kmem_cache_free() adds
> a bit of overhead:
> 	it adds overhead of function call and can't benefit from inlining
> 	(including removing unnnecessary part of function code) 

Hm, right.

> So... what about including slab_common.c in sl?b.c,
> so that compiler can treat sl?b.c and slab_common.c as a single translation unit?
> (or split kmalloc implementation into kmalloc.c and do similar thing?)

I don't know if that has a good precedent in the kernel. Maybe we can
postpone these more radical attempts to a later series.