linux-kernel - Re: [PATCH v2 00/10] evacuate struct page from the block layer, introduce __pfn

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Date:	Thu, 7 May 2015 10:43:01 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Dan Williams <dan.j.williams@...el.com>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Boaz Harrosh <boaz@...xistor.com>, Jan Kara <jack@...e.cz>,
	Mike Snitzer <snitzer@...hat.com>, Neil Brown <neilb@...e.de>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	Chris Mason <clm@...com>, Paul Mackerras <paulus@...ba.org>,
	"H. Peter Anvin" <hpa@...or.com>, Christoph Hellwig <hch@....de>,
	Alasdair Kergon <agk@...hat.com>,
	"linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
	Ingo Molnar <mingo@...nel.org>, Mel Gorman <mgorman@...e.de>,
	Matthew Wilcox <willy@...ux.intel.com>,
	Ross Zwisler <ross.zwisler@...ux.intel.com>,
	Rik van Riel <riel@...hat.com>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Jens Axboe <axboe@...nel.dk>, "Theodore Ts'o" <tytso@....edu>,
	"Martin K. Petersen" <martin.petersen@...cle.com>,
	Julia Lawall <Julia.Lawall@...6.fr>, Tejun Heo <tj@...nel.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH v2 00/10] evacuate struct page from the block layer,
 introduce __pfn_t

On Thu, May 7, 2015 at 9:03 AM, Dan Williams <dan.j.williams@...el.com> wrote:
>
> Ok, I'll keep thinking about this and come back when we have a better
> story about passing mmap'd persistent memory around in userspace.

Ok. And if we do decide to go with your kind of "__pfn" type, I'd
probably prefer that we encode the type in the low bits of the word
rather than compare against PAGE_OFFSET. On some architectures
PAGE_OFFSET is zero (admittedly probably not ones you'd care about),
but even on x86 it's a *lot* cheaper to test the low bit than it is to
compare against a big constant.

We know "struct page *" is supposed to be at least aligned to at least
"unsigned long", so you'd have two bits of type information (and we
could easily make it three). With "0" being a real pointer, so that
you can use the pointer itself without masking.

And the "hide type in low bits of pointer" is something we've done
quite a lot, so it's more "kernel coding style" anyway.

                            Linus
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