linux-kernel - Re: [PATCHv7 10/24] zram: add zlib compression backend support

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Message-ID: <gekqwhcpombpm2u3b4xl7zladuyzbxybeq5wcwt47k7tsgo4bh@rfrxaeqwzypi>
Date: Fri, 9 May 2025 10:38:14 +0900
From: Sergey Senozhatsky <senozhatsky@...omium.org>
To: Zaslonko Mikhail <zaslonko@...ux.ibm.com>
Cc: Sergey Senozhatsky <senozhatsky@...omium.org>, 
	Minchan Kim <minchan@...nel.org>, linux-kernel@...r.kernel.org, 
	Andrew Morton <akpm@...ux-foundation.org>, Ilya Leoshkevich <iii@...ux.ibm.com>, 
	Heiko Carstens <hca@...ux.ibm.com>, linux-s390@...r.kernel.org
Subject: Re: [PATCHv7 10/24] zram: add zlib compression backend support

Hello,

On (25/05/08 16:19), Zaslonko Mikhail wrote:
[..]
> > +#include "backend_deflate.h"
> > +
> > +/* Use the same value as crypto API */
> > +#define DEFLATE_DEF_WINBITS		11
> > +#define DEFLATE_DEF_MEMLEVEL		MAX_MEM_LEVEL
> > +
[..]
> The comment line says 'Use the same value as crypto API'...
> could you please clarify here? Are there some memory constraints?

When zram transitioned from Crypto API (scomp) to custom compression
API I picked the CryptoAPI deflate DEFLATE_DEF_WINBITS value:

crypto/deflate.c: DEFLATE_DEF_WINBITS	11

which is then passed to zlib_deflateInit2() and zlib_inflateInit2().

> I tried to build the kernel with DEFLATE_DEF_WINBITS set to 15 and
> verified that s390 hardware deflate acceleration works for zram devices
> with a deflate compression.

If we define it as 15 on non-s390 machines, will there be any
consequences?  Increased memory usage?  By how much?