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

From: Andrew Morton
Date: Fri Sep 21 2018 - 19:34:17 EST


On Thu, 20 Sep 2018 21:00:16 +0100 Aaron Tomlin <atomlin@xxxxxxxxxx> 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 ;)