linux-kernel - Re: [Intel-wired-lan] [PATCH 1/5] ethtool: use vmalloc

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Message-ID: <2ec36cd7-7378-4e44-894a-93008348a96a@molgen.mpg.de>
Date: Thu, 14 Aug 2025 08:28:42 +0200
From: Paul Menzel <pmenzel@...gen.mpg.de>
To: Qianfeng Rong <rongqianfeng@...o.com>
Cc: Tony Nguyen <anthony.l.nguyen@...el.com>,
 Przemek Kitszel <przemyslaw.kitszel@...el.com>,
 Andrew Lunn <andrew+netdev@...n.ch>, "David S. Miller"
 <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>,
 Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
 Alexei Starovoitov <ast@...nel.org>, Daniel Borkmann <daniel@...earbox.net>,
 Jesper Dangaard Brouer <hawk@...nel.org>,
 John Fastabend <john.fastabend@...il.com>,
 Stanislav Fomichev <sdf@...ichev.me>, intel-wired-lan@...ts.osuosl.org,
 netdev@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
 bpf@...r.kernel.org
Subject: Re: [Intel-wired-lan] [PATCH 1/5] ethtool: use vmalloc_array() to
 simplify code

Dear Quianfeng,

Thank you very much for your reply.

Am 14.08.25 um 06:05 schrieb Qianfeng Rong:
> 
> 在 2025/8/13 0:34, Paul Menzel 写道:

[…]

>> Am 12.08.25 um 15:32 schrieb Qianfeng Rong:
>>> Remove array_size() calls and replace vmalloc() with vmalloc_array() to
>>> simplify the code and maintain consistency with existing kmalloc_array()
>>> usage.
>>
>> You could build it without and with your patch and look if the assembler
>> code changes.
> 
> Very good point, the following experiment was done:
> //before apply patch:
> objdump -dSl --prefix-addresses fm10k_ethtool.o > original.dis
> 
> //after apply patch:
> objdump -dSl --prefix-addresses fm10k_ethtool.o > patched.dis
> 
> diff -u original.dis patched.dis | diffstat
> patched.dis | 1578 ... 1 file changed, 785 insertions(+), 793 deletions(-)
> 
> From the above results, we can see that the assembly instructions are
> reduced after applying the patch.
> 
> 
> #define array_size(a, b)    size_mul(a, b)
> 
> static inline size_t __must_check size_mul(size_t factor1, size_t factor2)
> {
>      size_t bytes;
> 
>      if (check_mul_overflow(factor1, factor2, &bytes))
>          return SIZE_MAX;
> 
>      return bytes;
> }
> 
> void *__vmalloc_array_noprof(size_t n, size_t size, gfp_t flags)
> {
>      size_t bytes;
> 
>      if (unlikely(check_mul_overflow(n, size, &bytes)))
>          return NULL;
>      return __vmalloc_noprof(bytes, flags);
> }
> 
> And from the code, array_size() will return SIZE_MAX after detecting
> overflow.  SIZE_MAX is passed to vmalloc for available memory
> verification before exiting and returning NULL. vmalloc_array()
> will directly return NULL after detecting overflow.

Awesome! Thank you for digging that up. Maybe something to add to the 
commit message. Maybe something like:

`vmalloc_array()` is also optimized better, resulting in less 
instructions being used, which can be verified with:

objdump -dSl --prefix-addresses <changed module>.o

>> Reviewed-by: Paul Menzel <pmenzel@...gen.mpg.de>

Kind regards,

Paul