linux-kernel - Re: [PATCH] pagemap: early return on unmapped areas

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Date:	Thu, 14 Jan 2010 11:02:11 -0600
From:	Matt Mackall <mpm@...enic.com>
To:	Wu Fengguang <fengguang.wu@...el.com>
Cc:	Andi Kleen <andi@...stfloor.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	"Rafael J. Wysocki" <rjw@...k.pl>
Subject: Re: [PATCH] pagemap: early return on unmapped areas

On Thu, 2010-01-14 at 16:52 +0800, Wu Fengguang wrote:
> On Thu, Jan 14, 2010 at 04:30:48PM +0800, Andi Kleen wrote:
> > > ---
> > > pagemap: early return on unmapped areas
> > 
> > Yes seems like a reasonable idea.
> 
> Thanks!
> 
> I just changed the ">" to ">=", because it happen to make a difference
> for cp/cat, which does 4k sized read:
> 
>         4k read buffer = 4k/8 pages = 512 pages = NR_PMD_PAGES
> 
> So the change to ">=" makes cp exit immediately (4k buffer matches
> exactly one PMD hole).
> 
> before
>         dd if=/proc/$$/pagemap of=/dev/null bs=4k
>         512+0 records in
>         512+0 records out
>         2097152 bytes (2.1 MB) copied, 0.0225796 s, 92.9 MB/s
> 
> after
>         dd if=/proc/$$/pagemap of=/dev/null bs=4k
>         0+0 records in
>         0+0 records out
>         0 bytes (0 B) copied, 2.8029e-05 s, 0.0 kB/s

Have you guys lost your minds?

I regularly used the exact standard tool you just broke above to test
pagemap when I was developing it. There's no doubt that this patch (or
any similarly misguided ones) WILL regress existing naive or not
intended to be portable 32-bit users. And if I was some poor future
bastard trying to write a program against pagemap with your patch, odds
of getting a concussion banging my head against the wall trying to
figure out its highly non-file-like behavior would be quite high. 

Why are we even still on this. There's no regression. LTP + pagemap +
64-bit has always behaved thus. Nor are large virtual files new. And
LTP's n00b assumption that it can just read all of any file in /proc has
always been wrong. LTP had the exact same problem on 64-bit /proc/kcore
(how long has that existed?) and finally fixed it in 2005.

-- 
http://selenic.com : development and support for Mercurial and Linux

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