linux-kernel - Re: [PATCH] checkpatch: warn that small allocs should be combined

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Date:   Sun, 13 Mar 2022 09:09:16 -0700
From:   Joe Perches <joe@...ches.com>
To:     trix@...hat.com, apw@...onical.com, dwaipayanray1@...il.com,
        lukas.bulwahn@...il.com
Cc:     linux-kernel@...r.kernel.org
Subject: Re: [PATCH] checkpatch: warn that small allocs should be combined

On Sun, 2022-03-13 at 07:08 -0700, trix@...hat.com wrote:
> From: Tom Rix <trix@...hat.com>
> 
> A memory allocation has overhead.  When a
> small allocation is made the overhead dominates.
> By combining the fixed sized small allocations
> with others, the memory usage can be reduced
> by eliminating the overhead of the small allocs.

This will generate false positives as small allocs are
sometimes required for usb dma.

How many of these "small allocs" _could_ be combined and under
what circumstance?

Can you show me a current example in the kernel where this
is useful?

> diff --git a/scripts/checkpatch.pl b/scripts/checkpatch.pl
[]
> @@ -7076,6 +7076,12 @@ sub process {
>  			     "$1 uses number as first arg, sizeof is generally wrong\n" . $herecurr);
>  		}
>  
> +# check for small allocs
> +		if ($line =~ /\b(?:kv|k|v)[zm]alloc\s*\(\s*(\d|sizeof\s*\([su](8|16|32)s*\))\s*,/) {
> +			WARN("SMALL_ALLOC",
> +			     "Small allocs should be combined\n" . $herecurr);
> +		}
> +

Couple more comments:

Anyone using vmalloc variants for a small alloc is confused.
What defines "small"?
Why would a single decimal like 8 be small, but say 16 would not be?

checkpatch has a couple of regexes that could be useful here

Maybe instead of sizeof(your regex) use

	sizeof\s*\(\s*(?:\d|$C90_int_types|$typeTypedefs)\s*,

as that will find more "small" uses of individual types like
"unsigned long", __s32, u_int_16, etc...