linux-kernel - Re: [PATCH 0/5] kstrdup optimization

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Message-ID: <CAMuHMdV74n3v81xaLRDN_Mn_QGg14yUkXNn6JYaGH4MGgLRM2A@mail.gmail.com>
Date:	Mon, 12 Jan 2015 21:45:58 +0100
From:	Geert Uytterhoeven <geert@...ux-m68k.org>
To:	Andrzej Hajda <a.hajda@...sung.com>
Cc:	Linux MM <linux-mm@...ck.org>,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	Kyungmin Park <kyungmin.park@...sung.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Andi Kleen <andi@...stfloor.org>, Andreas Mohr <andi@...as.de>,
	Mike Turquette <mturquette@...aro.org>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 0/5] kstrdup optimization

On Mon, Jan 12, 2015 at 10:18 AM, Andrzej Hajda <a.hajda@...sung.com> wrote:
> kstrdup if often used to duplicate strings where neither source neither
> destination will be ever modified. In such case we can just reuse the source
> instead of duplicating it. The problem is that we must be sure that
> the source is non-modifiable and its life-time is long enough.
>
> I suspect the good candidates for such strings are strings located in kernel
> .rodata section, they cannot be modifed because the section is read-only and
> their life-time is equal to kernel life-time.
>
> This small patchset proposes alternative version of kstrdup - kstrdup_const,
> which returns source string if it is located in .rodata otherwise it fallbacks
> to kstrdup.

It also introduces kfree_const(const void *x).

As kfree_const() has the exact same signature as kfree(), the risk of
accidentally passing pointers returned from kstrdup_const() to kfree() seems
high, which may lead to memory corruption if the pointer doesn't point to
allocated memory.

> To verify if the source is in .rodata function checks if the address is between
> sentinels __start_rodata, __end_rodata. I guess it should work with all
> architectures.
>
> The main patch is accompanied by four patches constifying kstrdup for cases
> where situtation described above happens frequently.
>
> As I have tested the patchset on mobile platform (exynos4210-trats) it saves
> 3272 string allocations. Since minimal allocation is 32 or 64 bytes depending
> on Kconfig options the patchset saves respectively about 100KB or 200KB of memory.

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds
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