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Date: Tue, 7 Jan 2020 15:30:59 +0100
From: David Sterba <dsterba@suse.cz>
Subject: Re: [PATCH v3 6/6] btrfs: Use larger zlib buffer for s390 hardware
 compression
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To: Eduard Shishkin <edward6@linux.ibm.com>
Cc: dsterba@suse.cz, Mikhail Zaslonko <zaslonko@linux.ibm.com>, Andrew Morton <akpm@linux-foundation.org>, Chris Mason <clm@fb.com>, Josef Bacik <josef@toxicpanda.com>, David Sterba <dsterba@suse.com>, Richard Purdie <rpurdie@rpsys.net>, Heiko Carstens <heiko.carstens@de.ibm.com>, Vasily Gorbik <gor@linux.ibm.com>, Christian Borntraeger <borntraeger@de.ibm.com>, Ilya Leoshkevich <iii@linux.ibm.com>, linux-s390@vger.kernel.org, linux-kernel@vger.kernel.org

On Tue, Jan 07, 2020 at 01:18:06AM +0100, Eduard Shishkin wrote:
> >> @@ -61,7 +64,17 @@ struct list_head *zlib_alloc_workspace(unsigned int level)
> >>   			zlib_inflate_workspacesize());
> >>   	workspace->strm.workspace = kvmalloc(workspacesize, GFP_KERNEL);
> >>   	workspace->level = level;
> >> -	workspace->buf = kmalloc(PAGE_SIZE, GFP_KERNEL);
> >> +	workspace->buf = NULL;
> >> +	if (zlib_deflate_dfltcc_enabled()) {
> >> +		workspace->buf = kmalloc(ZLIB_DFLTCC_BUF_SIZE,
> >> +					 __GFP_NOMEMALLOC | __GFP_NORETRY |
> >> +					 __GFP_NOWARN | GFP_NOIO);
> > Why do you use this wild GFP flag combination? I can understand NOWARN,
> > but why the others?
> 
> This addresses the following complaint about order 2 allocation with 
> GFP_KERNEL:
> https://lkml.org/lkml/2019/11/26/417
> 
> Below a fallback to a single page is implemented, as it was suggested.
> So, the initial (more costly) allocation should be made with minimal 
> aggression
> to allow the allocator fail. Otherwise, that fallback simply doesn't 
> make sense.
> Hence, such a combination.

I see, please add a comment explaining that.