From: Rob Landley <rob@landley.net>
Organization: Boundaries Unlimited
To: Marcelo Tosatti <marcelo.tosatti@cyclades.com>
Subject: Re: [PATCH] skip initramfs check
Date: Mon, 21 Nov 2005 09:04:46 -0600
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Cc: akpm@osdl.org, linux-kernel@vger.kernel.org,
       Matt Mackall <mpm@selenic.com>,
       Al Viro <viro@parcelfarce.linux.theplanet.co.uk>
References: <20051117141432.GD9753@logos.cnet> <200511210130.55774.rob@landley.net> <20051121062350.GA24381@logos.cnet>
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On Monday 21 November 2005 00:23, Marcelo Tosatti wrote:
> Hi Rob,
> > Query: is the problem that a big initramfs image is being unpacked more
> > than once, or is unpacking an empty initramfs image (134 bytes) causing a
> > significant delay?
>
> The problem is a big non-initramfs RAMDISK image (used for root mountpoint
> on this particular embedded platform), that is decompressed more than once:
>
> - during the initramfs check, which fails because it is not initramfs.
> - during the real RAMDISK decompression to memory.
>
> > I'm fairly certain that back in 1990 I could unzip 134 bytes on my 33 mhz
> > 386 running dos in a fraction of a second.  What's the use case here?
>
> So the issue is not the empty initramfs image (which BTW could probably
> be made unecessary?), but a 10Mb RAMDISK image being decompressed by a
> 48Mhz PPC, which takes quite a few seconds.
>
> Need to rework the patch to use a __setup option as Andrew suggested.

It sounds to me like is the initial check (which is just giving a thumbs 
up/thumbs down "is this an initramfs", correct?) only needs to decompress the 
first page or so of data to make this determination.  A quick glance at the 
code seems to imply it's just checking the header and the first entry, so 4k 
should be plenty for that.

Some variant of lib/zlib_inflate...  Ouch, bit of a mess there.  Hey Matt: you 
know this area.  Is it feasible to do some kind of:

deflate_init(whatever)
deflate_next_x_bytes(source *, dest *, length)

To grab a the first X bytes from the initramfs image?

Rob
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