linux-kernel - Re: RFC: prepare for struct scatterlist entries without page backing

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Date:	Fri, 14 Aug 2015 09:40:30 +1000
From:	Julian Calaby <julian.calaby@...il.com>
To:	Christoph Hellwig <hch@....de>
Cc:	Boaz Harrosh <boaz@...xistor.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	axboe@...nel.dk, linux-mips@...ux-mips.org,
	linux-ia64@...r.kernel.org, linux-nvdimm@...1.01.org,
	David Howells <dhowells@...hat.com>,
	sparclinux <sparclinux@...r.kernel.org>,
	Hans-Christian Egtvedt <egtvedt@...fundet.no>,
	linux-arch@...r.kernel.org, linux-s390@...r.kernel.org,
	x86@...nel.org, David Woodhouse <dwmw2@...radead.org>,
	Håvard Skinnemoen <hskinnemoen@...il.com>,
	linux-xtensa@...ux-xtensa.org, grundler@...isc-linux.org,
	Miao Steven <realmz6@...il.com>, alex.williamson@...hat.com,
	linux-metag@...r.kernel.org, Michal Simek <monstr@...str.eu>,
	linux-parisc@...r.kernel.org, vgupta@...opsys.com,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	linux-alpha@...r.kernel.org,
	Linux Media Mailing List <linux-media@...r.kernel.org>,
	linuxppc-dev@...ts.ozlabs.org
Subject: Re: RFC: prepare for struct scatterlist entries without page backing

Hi Christoph,

On Fri, Aug 14, 2015 at 12:35 AM, Christoph Hellwig <hch@....de> wrote:
> On Thu, Aug 13, 2015 at 09:37:37AM +1000, Julian Calaby wrote:
>> I.e. ~90% of this patch set seems to be just mechanically dropping
>> BUG_ON()s and converting open coded stuff to use accessor functions
>> (which should be macros or get inlined, right?) - and the remaining
>> bit is not flushing if we don't have a physical page somewhere.
>
> Which is was 90%.  By lines changed most actually is the diffs for
> the cache flushing.

I was talking in terms of changes made, not lines changed: by my
recollection, about a third of the patches didn't touch flush calls
and most of the lines changed looked like refactoring so that making
the flush call conditional would be easier.

I guess it smelled like you were doing lots of distinct changes in a
single patch and I got my numbers wrong.

>> Would it make sense to split this patch set into a few bits: one to
>> drop all the useless BUG_ON()s, one to convert all the open coded
>> stuff to accessor functions, then another to do the actual page-less
>> sg stuff?
>
> Without the ifs the BUG_ON() actually are useful to assert we
> never feed the sort of physical addresses we can't otherwise support,
> so I don't think that part is doable.

My point is that there's a couple of patches that only remove
BUG_ON()s, which implies that for that particular driver it doesn't
matter if there's a physical page or not, so therefore that code is
purely "documentation".

Thanks,

-- 
Julian Calaby

Email: julian.calaby@...il.com
Profile: http://www.google.com/profiles/julian.calaby/
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