netdev - Re: [PATCH RFC PKS/PMEM 57/58] nvdimm/pmem: Stray access protection for pmem->virt

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Date:   Fri, 9 Oct 2020 19:53:07 -0700
From:   John Hubbard <jhubbard@...dia.com>
To:     <ira.weiny@...el.com>, Andrew Morton <akpm@...ux-foundation.org>,
        "Thomas Gleixner" <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "Borislav Petkov" <bp@...en8.de>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>
CC:     <x86@...nel.org>, Dave Hansen <dave.hansen@...ux.intel.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Fenghua Yu <fenghua.yu@...el.com>, <linux-doc@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, <linux-nvdimm@...ts.01.org>,
        <linux-fsdevel@...r.kernel.org>, <linux-mm@...ck.org>,
        <linux-kselftest@...r.kernel.org>, <linuxppc-dev@...ts.ozlabs.org>,
        <kvm@...r.kernel.org>, <netdev@...r.kernel.org>,
        <bpf@...r.kernel.org>, <kexec@...ts.infradead.org>,
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Subject: Re: [PATCH RFC PKS/PMEM 57/58] nvdimm/pmem: Stray access protection
 for pmem->virt_addr

On 10/9/20 12:50 PM, ira.weiny@...el.com wrote:
> From: Ira Weiny <ira.weiny@...el.com>
> 
> The pmem driver uses a cached virtual address to access its memory
> directly.  Because the nvdimm driver is well aware of the special
> protections it has mapped memory with, we call dev_access_[en|dis]able()
> around the direct pmem->virt_addr (pmem_addr) usage instead of the
> unnecessary overhead of trying to get a page to kmap.
> 
> Signed-off-by: Ira Weiny <ira.weiny@...el.com>
> ---
>   drivers/nvdimm/pmem.c | 4 ++++
>   1 file changed, 4 insertions(+)
> 
> diff --git a/drivers/nvdimm/pmem.c b/drivers/nvdimm/pmem.c
> index fab29b514372..e4dc1ae990fc 100644
> --- a/drivers/nvdimm/pmem.c
> +++ b/drivers/nvdimm/pmem.c
> @@ -148,7 +148,9 @@ static blk_status_t pmem_do_read(struct pmem_device *pmem,
>   	if (unlikely(is_bad_pmem(&pmem->bb, sector, len)))
>   		return BLK_STS_IOERR;
>   
> +	dev_access_enable(false);
>   	rc = read_pmem(page, page_off, pmem_addr, len);
> +	dev_access_disable(false);

Hi Ira!

The APIs should be tweaked to use a symbol (GLOBAL, PER_THREAD), instead of
true/false. Try reading the above and you'll see that it sounds like it's
doing the opposite of what it is ("enable_this(false)" sounds like a clumsy
API design to *disable*, right?). And there is no hint about the scope.

And it *could* be so much more readable like this:

     dev_access_enable(DEV_ACCESS_THIS_THREAD);



thanks,
-- 
John Hubbard
NVIDIA