linux-kernel - Re: [PATCH 12/31] huge tmpfs: extend get_user_pages

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Date:	Wed, 13 Apr 2016 10:58:49 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Hugh Dickins <hughd@...gle.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Andres Lagar-Cavilla <andreslc@...gle.com>,
	Yang Shi <yang.shi@...aro.org>, Ning Qu <quning@...il.com>,
	Ralf Baechle <ralf@...ux-mips.org>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	David Miller <davem@...emloft.net>,
	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
	linux-mm@...ck.org
Subject: Re: [PATCH 12/31] huge tmpfs: extend get_user_pages_fast to shmem pmd


* Hugh Dickins <hughd@...gle.com> wrote:

> > >  arch/mips/mm/gup.c  |   15 ++++++++++++++-
> > >  arch/s390/mm/gup.c  |   19 ++++++++++++++++++-
> > >  arch/sparc/mm/gup.c |   19 ++++++++++++++++++-
> > >  arch/x86/mm/gup.c   |   15 ++++++++++++++-
> > >  mm/gup.c            |   19 ++++++++++++++++++-
> > >  5 files changed, 82 insertions(+), 5 deletions(-)
> ...

> > Looks like there are two main variants - so these kinds of repetitive patterns 
> > very much call for some sort of factoring out of common code, right?
> 
> Hmm.  I'm still struggling between the two extremes, of
> 
> (a) agreeing completely with you, and saying, yeah, I'll take on the job
>     of refactoring every architecture's get_user_pages_as_fast_as_you_can(),
>     without much likelihood of testing more than one,
> 
> and
> 
> (b) running a mile, and pointing out that we have a tradition of using
>     arch/x86/mm/gup.c as a template for the others, and here I've just
>     added a few more lines to that template (which never gets built more
>     than once into any kernel).
> 
> Both are appealing in their different ways, but I think you can tell
> which I'm leaning towards...
> 
> Honestly, I am still struggling between those two; but I think the patch
> as it stands is one thing, and cleanup for commonality should be another
> however weaselly that sounds ("I'll come back to it" - yeah, right).

Yeah, so my worry is this: your patch for example roughly doubles the algorithmic 
complexity of mm/gup.c and arch/*/mm/gup.c's ::gup_huge_pmd().

And you want this to add a new feature!

So it really looks like to me this is the last sane chance to unify cheaply, then 
add the feature you want. Everyone else in the future will be able to refer to 
your example to chicken out! ;-)

Thanks,

	Ingo