netdev - Re: [PATCH net 1/1] s390/ism: fix receive message buffer allocation

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Date: Fri, 5 Apr 2024 08:49:19 +0200
From: Christoph Hellwig <hch@....de>
To: Gerd Bayer <gbayer@...ux.ibm.com>
Cc: Paolo Abeni <pabeni@...hat.com>, Wenjia Zhang <wenjia@...ux.ibm.com>,
	Wen Gu <guwen@...ux.alibaba.com>,
	Heiko Carstens <hca@...ux.ibm.com>, pasic@...ux.ibm.com,
	schnelle@...ux.ibm.com, linux-s390@...r.kernel.org,
	netdev@...r.kernel.org, Alexandra Winter <wintera@...ux.ibm.com>,
	Thorsten Winkler <twinkler@...ux.ibm.com>,
	Vasily Gorbik <gor@...ux.ibm.com>,
	Alexander Gordeev <agordeev@...ux.ibm.com>,
	Christian Borntraeger <borntraeger@...ux.ibm.com>,
	Sven Schnelle <svens@...ux.ibm.com>, Christoph Hellwig <hch@....de>
Subject: Re: [PATCH net 1/1] s390/ism: fix receive message buffer allocation

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.