linux-kernel - Re: [PATCH] x86/asm/memcpy_mcsafe: Fix copy_to_user

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Message-ID: <20180705070256.GA27603@gmail.com>
Date:   Thu, 5 Jul 2018 09:02:56 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     Al Viro <viro@...iv.linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andy Lutomirski <luto@...capital.net>,
        Borislav Petkov <bp@...en8.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Tony Luck <tony.luck@...el.com>,
        Ross Zwisler <ross.zwisler@...ux.intel.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        X86 ML <x86@...nel.org>
Subject: Re: [PATCH] x86/asm/memcpy_mcsafe: Fix copy_to_user_mcsafe()
 exception handling


* Dan Williams <dan.j.williams@...el.com> wrote:

> On Wed, Jul 4, 2018 at 3:38 PM, Al Viro <viro@...iv.linux.org.uk> wrote:
> > On Tue, Jul 03, 2018 at 10:30:40AM +0200, Ingo Molnar wrote:
> >>
> >> * Dan Williams <dan.j.williams@...el.com> wrote:
> >>
> >> > Hi Ingo,
> >> >
> >> > Here is an additional copy_to_iter_mcsafe() fix to address the crash
> >> > reported by Ross. This now passes xfstests:generic/323 on my system.
> >>
> >> The lib/iov_iter fix would need an Acked-by from Al.
> >
> > I can live with that; I would really like to see some documentation on
> > the copy_to_iter_mcsafe(), but that's a separate story.  Incidentally,
> > are there any expectations of other callers appearing, or is that
> > (and copy_from_iter_flushcache()) YASingleConsumerAPI?
> 
> The current cpu architectural detail preventing conversion of the
> standard copy_to_iter() path to use the mcsafe flavor is that we can't
> use REP MOV for fast copies and instead need to use a software loop so
> that any exceptions are recoverable. When / if that is addressed, and
> there is no performance difference between the two, it might make
> sense to convert more users.
> 
> The _flushcache flavor, however, will likely stay limited to a single
> consumer for the persistent memory use case.

Could you please add the API documentation Al asked for, and update the changlog 
with the acks and tested-by's that people gave, and send out a v2 series?

Thanks!

	Ingo