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

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Date:	Wed, 6 Apr 2016 19:53:57 -0700 (PDT)
From:	Hugh Dickins <hughd@...gle.com>
To:	Ingo Molnar <mingo@...nel.org>
cc:	Hugh Dickins <hughd@...gle.com>,
	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

On Wed, 6 Apr 2016, Ingo Molnar wrote:
> * Hugh Dickins <hughd@...gle.com> wrote:
> 
> > ---
> > Cc'ed to arch maintainers as an FYI: this patch is not expected to
> > go into the tree in the next few weeks, and depends upon a PageTeam
> > definition not yet available outside this huge tmpfs patchset.
> > Please refer to linux-mm or linux-kernel for more context.

Actually, Andrew took it and the rest into mmotm yesterday, to give them
better exposure through linux-next, so they should appear there soon.

> > 
> >  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(-)
...
> 
> Ouch!

Oh sorry, I didn't mean to hurt you ;)

> 
> 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).

Hugh

> 
> Then the fix could be applied to the common portion(s) only, which will cut down 
> this gigantic diffstat:
> 
>   >  5 files changed, 82 insertions(+), 5 deletions(-)
> 
> Thanks,
> 
> 	Ingo