linux-kernel - Re: [PATCH v3 06/11] scatterlist: support "page-less" (__pfn

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Message-ID: <CAA9_cmeD0245sggWkKnK9ur2q3fAYH3+Cr_ddQgKWfcKo1OtEQ@mail.gmail.com>
Date:	Sat, 23 May 2015 09:41:45 -0700
From:	Dan Williams <dan.j.williams@...el.com>
To:	"hch@....de" <hch@....de>
Cc:	"axboe@...nel.dk" <axboe@...nel.dk>,
	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	"riel@...hat.com" <riel@...hat.com>,
	"linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	"david@...morbit.com" <david@...morbit.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"mingo@...nel.org" <mingo@...nel.org>,
	"axboe@...com" <axboe@...com>,
	"dmaengine@...r.kernel.org" <dmaengine@...r.kernel.org>,
	"mgorman@...e.de" <mgorman@...e.de>,
	"j.glisse@...il.com" <j.glisse@...il.com>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>
Subject: Re: [PATCH v3 06/11] scatterlist: support "page-less" (__pfn_t only) entries

On Sat, May 23, 2015 at 7:12 AM, hch@....de <hch@....de> wrote:
> On Wed, May 13, 2015 at 06:35:55PM +0000, Williams, Dan J wrote:
>> Jens, I'm wondering if you want to take this series(.) as patches or
>> prepare a git branch to pull?
>
> Honestly I don't think it should go anyway.  It makes a big mess of
> a structure without providing a real user for it.  Given how we are
> using the bio_vec for in-kernel page based I/O these days it seems
> like a very dangerous idea.

There's nothing dangerous about the __pfn_t conversion of the block
layer in the !CONFIG_DEV_PFN case a __pfn_t based bio_vec is
bit-for-bit identical to a struct page based bio_vec.  However, you're
right, I can't make the same claim about a scatterlist before and
after the change.

Hmm, we're missing a pfn-only block I/O user and we're missing the
second half of the implementation that provides __pfn_t_to_page() for
persistent memory.  I'm looking to have a solution __pfn_t_to_page()
shortly, maybe that will allow the scatterlist changes to be
skipped...  we'll see.
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