linux-kernel - Re: [PATCH v2 6/6] btrfs: Use larger zlib buffer for s390 hardware compression

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Date:   Fri, 13 Dec 2019 18:35:27 +0100
From:   David Sterba <dsterba@...e.cz>
To:     Zaslonko Mikhail <zaslonko@...ux.ibm.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Chris Mason <clm@...com>, Josef Bacik <josef@...icpanda.com>,
        David Sterba <dsterba@...e.com>, linux-btrfs@...r.kernel.org,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Eduard Shishkin <edward6@...ux.ibm.com>,
        linux-s390@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 6/6] btrfs: Use larger zlib buffer for s390 hardware
 compression

On Fri, Dec 13, 2019 at 05:10:10PM +0100, Zaslonko Mikhail wrote:
> Hello,
> 
> Could you please review the patch for btrfs below.
> 
> Apart from falling back to 1 page, I have set the condition to allocate 
> 4-pages zlib workspace buffer only if s390 Deflate-Conversion facility
> is installed and enabled. Thus, it will take effect on s390 architecture
> only.
> 
> Currently in zlib_compress_pages() I always copy input pages to the workspace
> buffer prior to zlib_deflate call. Would that make sense, to pass the page
> itself, as before, based on the workspace buf_size (for 1-page buffer)?

Doesn't the copy back and forth kill the improvements brought by the
hw supported decompression?

> As for calling zlib_deflate with Z_FINISH flush parameter in a loop until
> Z_STREAM_END is returned, that comes in agreement with the zlib manual.

The concerns are about zlib stream that take 4 pages on input and on the
decompression side only 1 page is available for the output. Ie. as if
the filesystem was created on s390 with dflcc then opened on x86 host.
The zlib_deflate(Z_FINISH) happens on the compresission side.