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	id 9358268D07; Fri,  5 Apr 2024 08:49:19 +0200 (CEST)
Date: Fri, 5 Apr 2024 08:49:19 +0200
From: Christoph Hellwig <hch@lst.de>
To: Gerd Bayer <gbayer@linux.ibm.com>
Cc: Paolo Abeni <pabeni@redhat.com>, Wenjia Zhang <wenjia@linux.ibm.com>,
	Wen Gu <guwen@linux.alibaba.com>,
	Heiko Carstens <hca@linux.ibm.com>, pasic@linux.ibm.com,
	schnelle@linux.ibm.com, linux-s390@vger.kernel.org,
	netdev@vger.kernel.org, Alexandra Winter <wintera@linux.ibm.com>,
	Thorsten Winkler <twinkler@linux.ibm.com>,
	Vasily Gorbik <gor@linux.ibm.com>,
	Alexander Gordeev <agordeev@linux.ibm.com>,
	Christian Borntraeger <borntraeger@linux.ibm.com>,
	Sven Schnelle <svens@linux.ibm.com>, Christoph Hellwig <hch@lst.de>
Subject: Re: [PATCH net 1/1] s390/ism: fix receive message buffer allocation
Message-ID: <20240405064919.GA3788@lst.de>
References: <20240328154144.272275-1-gbayer@linux.ibm.com> <20240328154144.272275-2-gbayer@linux.ibm.com> <68ce59955f13751b3ced82cd557b069ed397085a.camel@redhat.com> <cb7b036b4d3db02ab70d17ee83e6bc4f2df03171.camel@linux.ibm.com>
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User-Agent: Mutt/1.5.17 (2007-11-01)

On Thu, Apr 04, 2024 at 01:10:20PM +0200, Gerd Bayer wrote:
> > Why can't you use get_free_pages() (or similar) here? (possibly
> > rounding up to the relevant page_aligned size). 
> 
> Thanks Paolo for your suggestion. However, I wanted to stay as close to
> the implementation pre [1] - that used to use __GFP_COMP, too. I'd
> rather avoid to change interfaces from "cpu_addr" to "struct page*" at
> this point. In the long run, I'd like to drop the requirement for

The right interface actually is to simply use folio_alloc, which adds
__GFP_COMP and is a fully supported and understood interface. You can
just convert the folio to a kernel virtual address using folio_address()
right after allocating it.

(get_free_pages also retunrs a kernel virtual address, just awkwardly as
an unsigned long. In doubt don't use this interface for new code..)

> compound pages entirely, since that *appears* to exist primarily for a
> simplified handling of the interface to splice_to_pipe() in
> net/smc/smc_rx.c. And of course there might be performance
> implications...

While compounds pages might sound awkward, they are the new normal in
form of folios.  So just use folios.