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

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Date:	Tue, 13 Jan 2015 15:48:49 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:	Andrzej Hajda <a.hajda@...sung.com>, 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>
Subject: Re: [PATCH 0/5] kstrdup optimization

On Mon, 12 Jan 2015 21:45:58 +0100 Geert Uytterhoeven <geert@...ux-m68k.org> wrote:

> 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.

Yes, it's an ugly little patchset.  But 100-200k of memory is hard to
argue with, and I'm not seeing a practical way of getting those savings
with a cleaner approach.

Hopefully a kfree(rodata-address) will promptly oops, but I haven't
tested that and it presumably depends on which flavour of
slab/sleb/slib/slob/slub you're using.

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