Re: RFC: prepare for struct scatterlist entries without page backing

From: Julian Calaby <julian.calaby@gmail.com>
To: Boaz Harrosh <boaz@plexistor.com>, Christoph Hellwig <hch@lst.de>
Cc: "Linus Torvalds" <torvalds@linux-foundation.org>,
	axboe@kernel.dk, linux-mips@linux-mips.org,
	linux-ia64@vger.kernel.org, linux-nvdimm@ml01.01.org,
	"David Howells" <dhowells@redhat.com>,
	sparclinux <sparclinux@vger.kernel.org>,
	"Hans-Christian Egtvedt" <egtvedt@samfundet.no>,
	linux-arch@vger.kernel.org, linux-s390@vger.kernel.org,
	x86@kernel.org, "David Woodhouse" <dwmw2@infradead.org>,
	"Håvard Skinnemoen" <hskinnemoen@gmail.com>,
	linux-xtensa@linux-xtensa.org, grundler@parisc-linux.org,
	realmz6@gmail.com, alex.williamson@redhat.com,
	linux-metag@vger.kernel.org, "Michal Simek" <monstr@monstr.eu>,
	linux-parisc@vger.kernel.org, vgupta@synopsys.com,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	linux-alpha@vger.kernel.org,
	"Linux Media Mailing List" <linux-media@vger.kernel.org>,
	linuxppc-dev@lists.ozlabs.org
Subject: Re: RFC: prepare for struct scatterlist entries without page backing
Date: Thu, 13 Aug 2015 09:37:37 +1000	[thread overview]
Message-ID: <CAGRGNgUKkaPnyvn30DXyNpdiXQzS6J=1+mQ3ick8C8=bhx_RHA@mail.gmail.com> (raw)
In-Reply-To: <55CB3F47.3000902@plexistor.com>

Hi,

On Wed, Aug 12, 2015 at 10:42 PM, Boaz Harrosh <boaz@plexistor.com> wrote:
> On 08/12/2015 10:05 AM, Christoph Hellwig wrote:
>> It turns out most DMA mapping implementation can handle SGLs without
>> page structures with some fairly simple mechanical work.  Most of it
>> is just about consistently using sg_phys.  For implementations that
>> need to flush caches we need a new helper that skips these cache
>> flushes if a entry doesn't have a kernel virtual address.
>>
>> However the ccio (parisc) and sba_iommu (parisc & ia64) IOMMUs seem
>> to be operate mostly on virtual addresses.  It's a fairly odd concept
>> that I don't fully grasp, so I'll need some help with those if we want
>> to bring this forward.
>>
>> Additional this series skips ARM entirely for now.  The reason is
>> that most arm implementations of the .map_sg operation just iterate
>> over all entries and call ->map_page for it, which means we'd need
>> to convert those to a ->map_pfn similar to Dan's previous approach.
>>
>
[snip]
>
> It is a bit of work but is worth while, and accelerating tremendously
> lots of workloads. Not like this abomination which only branches
> things more and more, and making things fatter and slower.

As a random guy reading a big bunch of patches on code I know almost
nothing about, parts of this comment really resonated with me:
overall, we seem to be adding a lot of if statements to code that
appears to be in a hot path.

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.

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?

Thanks,

-- 
Julian Calaby

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