linux-kernel - Re: [PATCH 2/2] x86: Only do a single page fault for copy_from_user

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Date:	Mon, 29 Sep 2014 08:26:46 -0700
From:	Andi Kleen <ak@...ux.intel.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Andi Kleen <andi@...stfloor.org>, dave@...1.net,
	linux-kernel@...r.kernel.org, mingo@...nel.org, eranian@...gle.com,
	x86@...nel.org, torvalds@...ux-foundation.org,
	Vitaly Mayatskikh <v.mayatskih@...il.com>
Subject: Re: [PATCH 2/2] x86: Only do a single page fault for
 copy_from_user_nmi

> For now, changing the semantics of the function seems like a sure way to
> fail in the future though.

I doubt it. Nearly nobody uses the exact return value semantics.

(iirc it's mostly write() and some bizarre code in mount)

In fact it's a regular mistake to assume it returns -errno.

> > In theory we could also duplicate the whole copy_*_ path for cases
> > where the caller doesn't care about the exact bytes. But that
> > seems overkill for just this issue, and I'm not sure anyone
> > else cares about how fast this is. The simpler check works
> > as well for now.
> 
> So I don't get that code, but why not fix it in general? Taking two
> faults seems silly.

It's really complicated to reconstruct the exact bytes, as an optimized
memcpy is very complicated and has a lot of corner cases. 

I tried it originally when writing the original copy function, but
failed. That is why people came up later with this two-fault
scheme.

I think two fault is fine for most cases, just not for NMIs.

-Andi

-- 
ak@...ux.intel.com -- Speaking for myself only
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