linux-kernel - Re: [patch] x86, mm: pass in 'total' to __copy_from_user

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Date:	Sat, 28 Feb 2009 22:49:56 +1100
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Salman Qazi <sqazi@...gle.com>, davem@...emloft.net,
	linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>, Andi Kleen <andi@...stfloor.org>
Subject: Re: [patch] x86, mm: pass in 'total' to __copy_from_user_*nocache()

On Saturday 28 February 2009 19:29:22 Ingo Molnar wrote:
> * Nick Piggin <nickpiggin@...oo.com.au> wrote:
> > On Thursday 26 February 2009 03:04:22 Linus Torvalds wrote:

> > > And we pretty much know that the only time we will ever really care
> > > about the nontemporal case is with big writes - where the "edge
> > > effects" essentially become total noise.
> >
> > I guess so. I wouldn't mind just doing cached stores all the
> > time for the reasons you say.
> >
> > But whatever. If it ever becomes *really* important, I guess
> > we can flag this kind of behaviour from userspace.
>
> Important question: is there a standing NAK for the 'total'
> parameter addition patch i did? You requested it and Linus didnt
> like it ... and i've measured it and it's just a single
> instruction in the whole kernel so it did not seem to be too bad
> to me.

Well I didn't request it so much as just being concerned about hard
transitions in behaviour in general, and also the lack of an overall
picture (which this patch improves a little, but doesn't solve --
only way to really do it is with explicit flags).


> It might be wrong on the principle though, so will revert it if
> needed, before it spreads into too many topics.

I would say if Linus didn't like it, revert it.

Do you use NFS with 1500 byte packets with a high speed network and
really fast disks (or ramdisk)? Then maybe you can measure vectored
write speedup with your patch over Salman's more basic one :)

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