linux-kernel - Re: [PATCH] lib/vsprintf.c: increase the size of the field

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Message-ID: <20150910154706.GG8114@mtj.duckdns.org>
Date:	Thu, 10 Sep 2015 11:47:06 -0400
From:	Tejun Heo <tj@...nel.org>
To:	Joe Perches <joe@...ches.com>
Cc:	Rasmus Villemoes <linux@...musvillemoes.dk>,
	Maurizio Lombardi <mlombard@...hat.com>,
	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] lib/vsprintf.c: increase the size of the field_width
 variable

Hello, Joe.

On Thu, Sep 10, 2015 at 08:41:25AM -0700, Joe Perches wrote:
> On Thu, 2015-09-10 at 10:36 -0400, Tejun Heo wrote:
> > On Wed, Sep 09, 2015 at 12:26:39PM -0700, Joe Perches wrote:
> > > > > %*pb is meant for smallish bitmaps, not big ones.
> []
> > The use case isn't from me, but why not?
> 
> Imagine the output of the 500k bitmap if every other
> bit is set.

Yeah, but the caller still should be able to call, say, scnprintf()
with a limited buffer and get the output till the end of the buffer
along with the indication that the output has been truncated.

> %*pb isn't capable of multiple line output and
> seq_printf output would also fail as it uses k.alloc
> memory not vmalloc.

Heh, and it should fail.

> I think that a more limited mechanism might be to use a
> multiple line oriented function like print_hex_debug
> and not try to emit the entire thing in a single go.

I'm not that worried about the berserk cases which try to print a
really long output on consoles but the subtle failure modes are
worrying.

> > Why are we even copying the struct on invocations?
> > Only some functions modify the values after all.
> > We might as well pass around pointer to the struct
> > and let the callees wihch modify them copy the
> > fields in local vars like normal functions.
> 
> You are of course welcome and able to change it.
> 
> btw: the current implementation has a limitation
> on 32 bit arches as it uses an int argument for the
> unsigned long count of bits in a bitmap.
> 
> That's a bitmap that should not be printed anyway.

That's 256Mbytes of bitmap.  I don't think we need to worry about that
at the moment.

Thanks.

-- 
tejun
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