linux-kernel - Re: [PATCH] x86, pmem: fix broken __copy_user

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Date:   Fri, 7 Apr 2017 16:51:15 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     "Kani, Toshimitsu" <toshi.kani@....com>
Cc:     "linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "jmoyer@...hat.com" <jmoyer@...hat.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "hch@....de" <hch@....de>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>,
        "viro@...iv.linux.org.uk" <viro@...iv.linux.org.uk>,
        "x86@...nel.org" <x86@...nel.org>,
        "mawilcox@...rosoft.com" <mawilcox@...rosoft.com>,
        "hpa@...or.com" <hpa@...or.com>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "ross.zwisler@...ux.intel.com" <ross.zwisler@...ux.intel.com>,
        "jack@...e.cz" <jack@...e.cz>
Subject: Re: [PATCH] x86, pmem: fix broken __copy_user_nocache cache-bypass assumptions

On Fri, Apr 7, 2017 at 10:41 AM, Kani, Toshimitsu <toshi.kani@....com> wrote:
> On Thu, 2017-04-06 at 13:59 -0700, Dan Williams wrote:
>> Before we rework the "pmem api" to stop abusing __copy_user_nocache()
>> for memcpy_to_pmem() we need to fix cases where we may strand dirty
>> data in the cpu cache. The problem occurs when copy_from_iter_pmem()
>> is used for arbitrary data transfers from userspace. There is no
>> guarantee that these transfers, performed by dax_iomap_actor(), will
>> have aligned destinations or aligned transfer lengths. Backstop the
>> usage __copy_user_nocache() with explicit cache management in these
>> unaligned cases.
>>
>> Yes, copy_from_iter_pmem() is now too big for an inline, but
>> addressing that is saved for a later patch that moves the entirety of
>> the "pmem api" into the pmem driver directly.
>
> The change looks good to me.  Should we also avoid cache flushing in
> the case of size=4B & dest aligned by 4B?

Yes, since you fixed the 4B aligned case we should skip cache flushing
in that case. I'll send a v2.