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

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Date:   Wed, 4 Jul 2018 16:02:31 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     Ingo Molnar <mingo@...nel.org>,
        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

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.