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To: linux-kernel@vger.kernel.org
From: "H. Peter Anvin" <hpa@zytor.com>
Subject: Re: libz, libbz2, ramfs and cramfs
Date: 18 Oct 2001 10:33:36 -0700
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Followup to:  <15310.51180.802846.33348@cargo.ozlabs.ibm.com>
By author:    Paul Mackerras <paulus@samba.org>
In newsgroup: linux.dev.kernel
> 
> ppp_deflate doesn't use next_out = NULL in this case, but it does use
> next_out = NULL when we are compressing a packet and the compressed
> packet turns out to be larger than the uncompressed.  With deflate
> there is a limit on how much larger the compressed packet would be, so
> it would be possible to give it a small extra buffer on the stack
> instead of using next_out = NULL.
> 

Discarding data is an important operation -- in zisofs that can happen
on a page lock conflict; I just use a dummy page for that.

> If we were going to standardize on a newer zlib in the kernel, I could
> change ppp_deflate to cope with that without too much pain, I think.
> The main thing I would want to add is a way to check what state the
> decompressor is in at the end of each packet - we want
> strm->state->blocks->mode == LENS at that point, which is not
> something that can be checked using the existing zlib interface.

The big issue is memory management.  I *think* the memory policy I
implemented in inflate_fs would work for PPP (you have to provide a
memory area of sufficient size, about 40K, at the time you open a
stream.  In the case of PPP this could probably just be vmalloc()'d at
the time the interface is created.  In the fs case, this is not
acceptable; rather, the filesystem in question has to maintain a
preallocation of memory and mutex it properly.)

	-hpa
-- 
<hpa@transmeta.com> at work, <hpa@zytor.com> in private!
"Unix gives you enough rope to shoot yourself in the foot."
http://www.zytor.com/~hpa/puzzle.txt	<amsp@zytor.com>
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