linux-kernel - Re: [PATCH v2] slub: extend slub debug to handle multiple slabs

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Date:   Fri, 21 Sep 2018 16:34:12 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Aaron Tomlin <atomlin@...hat.com>
Cc:     cl@...ux.com, penberg@...nel.org, rientjes@...gle.com,
        iamjoonsoo.kim@....com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] slub: extend slub debug to handle multiple slabs

On Thu, 20 Sep 2018 21:00:16 +0100 Aaron Tomlin <atomlin@...hat.com> wrote:

> Extend the slub_debug syntax to "slub_debug=<flags>[,<slub>]*", where <slub>
> may contain an asterisk at the end.  For example, the following would poison
> all kmalloc slabs:
> 
> 	slub_debug=P,kmalloc*
> 
> and the following would apply the default flags to all kmalloc and all block IO
> slabs:
> 
> 	slub_debug=,bio*,kmalloc*
> 
> Please note that a similar patch was posted by Iliyan Malchev some time ago but
> was never merged:
> 
> 	https://marc.info/?l=linux-mm&m=131283905330474&w=2

Fair enough, I guess.

> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -1283,9 +1283,37 @@ slab_flags_t kmem_cache_flags(unsigned int object_size,
>  	/*
>  	 * Enable debugging if selected on the kernel commandline.
>  	 */

The above comment is in a strange place.  Can we please move it to
above the function definition in the usual fashion?  And make it
better, if anything seems to be missing.

> -	if (slub_debug && (!slub_debug_slabs || (name &&
> -		!strncmp(slub_debug_slabs, name, strlen(slub_debug_slabs)))))
> -		flags |= slub_debug;
> +
> +	char *end, *n, *glob;

`end' and `glob' could be local to the loop which uses them, which I
find a bit nicer.

`n' is a rotten identifier.  Can't we think of something which
communicates meaning?

> +	int len = strlen(name);
> +
> +	/* If slub_debug = 0, it folds into the if conditional. */
> +	if (!slub_debug_slabs)
> +		return flags | slub_debug;

If we take the above return, the call to strlen() was wasted cycles. 
Presumably gcc is smart enough to prevent that, but why risk it.

> +	n = slub_debug_slabs;
> +	while (*n) {
> +		int cmplen;
> +
> +		end = strchr(n, ',');
> +		if (!end)
> +			end = n + strlen(n);
> +
> +		glob = strnchr(n, end - n, '*');
> +		if (glob)
> +			cmplen = glob - n;
> +		else
> +			cmplen = max(len, (int)(end - n));

max_t() exists for this.  Or maybe make `len' size_t, but I expect that
will still warn - that subtraction returns a ptrdiff_t, yes?

> +
> +		if (!strncmp(name, n, cmplen)) {
> +			flags |= slub_debug;
> +			break;
> +		}
> +
> +		if (!*end)
> +			break;
> +		n = end + 1;
> +	}

The code in this loop hurts my brain a bit. I hope it's correct ;)