linux-kernel - Re: [RFC] memcpy_nocache() and memcpy

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Date:   Tue, 3 Jan 2017 16:57:40 -0800
From:   Dan Williams <dan.j.williams@...el.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Al Viro <viro@...iv.linux.org.uk>,
        "Elliott, Robert (Persistent Memory)" <elliott@....com>,
        Boaz Harrosh <boaz@...xistor.com>,
        "linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
        "Moreno, Oliver" <oliver.moreno@....com>,
        "x86@...nel.org" <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "boylston@...romesa.net" <boylston@...romesa.net>
Subject: Re: [RFC] memcpy_nocache() and memcpy_writethrough()

On Tue, Jan 3, 2017 at 3:46 PM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> On Tue, Jan 3, 2017 at 3:22 PM, Al Viro <viro@...iv.linux.org.uk> wrote:
>>
>> 1) memcpy_to_pmem() seems to rely upon the __copy_from_user_nocache()
>> having only used movnt; it does not attempt clwb at all.
>>
>> 2) __copy_from_user_nocache() for short copies does not use movnt at all.
>> In that case neither sfence nor clwb is issued.
>
> Quite frankly, the whole "memcpy_nocache()" idea or (ab-)using
> copy_user_nocache() just needs to die. It's idiotic.
>
> As you point out, it's also fundamentally buggy crap.
>
> Throw it away. There is no possible way this is ever valid or
> portable. We're not going to lie and claim that it is.
>
> If some driver ends up using "movnt" by hand, that is up to that
> *driver*. But no way in hell should we care about this one whit in the
> sense of <linux/uaccess.h>. Get rid of that shit.
>
> So Al - just ignore this whole issue. It's not your headache. Any code
> that tries to depend on some non-caching memcpy is terminally buggy,
> and those code paths need to fix themselves, not ask others to fix
> their braindamage for them.

It's not Al's headache and our usage of __copy_from_user_nocache is a
blatant abuse, but the discussion is worth having because this is not
the first time we've struggled with the pmem api and the balance
between what functionality should be in fs/dax.c vs
drivers/nvdimm/pmem.c.

The stumbling block in the past to relegating all pmem accesses to the
driver is not wanting to further expand block_device_operations with
more dax specifics beyond the ->direct_access() operation we already
have.

I can think of gross ways of moving dax_iomap_actor() into the driver,
but perhaps less gross than burdening the uaccess.h maintainer with
pmem abuses.

This would also allow us to drop the needless cache maintenance for
dax capable drivers like brd that are fronting volatile memory.