Re: [PATCH 2/2] dax: fix bdev NULL pointer dereferences

[Dan Williams <dan.j.williams@intel.com>]

On Sun, Jan 31, 2016 at 10:07 AM, Matthew Wilcox <willy@linux.intel.com> wrote:
> On Sun, Jan 31, 2016 at 08:38:20AM -0800, Dan Williams wrote:
>> On Sun, Jan 31, 2016 at 2:55 AM, Matthew Wilcox <willy@linux.intel.com> wrote:
>> > On Sat, Jan 30, 2016 at 11:12:12PM -0700, Ross Zwisler wrote:
>> >> Is there a reason to store pnfs instead of kaddrs in the radix tree?
>> >
>> > Once ARM, MIPS and SPARC get supported, they're going to need temporary
>> > kernel addresses assigned to PFNs rather than permanent ones.  Also,
>> > it'll be easier for teardown to delete PFNs associated with a particular
>> > device than kaddrs associated with a particular device.  And it lets
>> > us support more persistent memory on a 32-bit machine (also on a 64-bit
>> > machine, but that's mostly theoretical)
>> >
>> > +/*
>> > + * DAX uses the 'exceptional' entries to store PFNs in the radix tree.
>> > + * Bit 0 is clear (the radix tree uses this for its own purposes).  Bit
>> > + * 1 is set (to indicate an exceptional entry).  Bits 2 & 3 are PFN_DEV
>> > + * and PFN_MAP.  The top two bits denote the size of the entry (PTE, PMD,
>> > + * PUD, one reserved).  That leaves us 26 bits on 32-bit systems and 58
>> > + * bits on 64-bit systems, able to address 256GB and 1024EB respectively.
>> > + */
>> >
>> > It's also pretty cheap to look up the kaddr from the pfn, at least on
>> > 64-bit architectures without cache aliasing problems:
>> >
>> > +static void *dax_map_pfn(pfn_t pfn, unsigned long index)
>> > +{
>> > +       preempt_disable();
>> > +       pagefault_disable();
>> > +       return pfn_to_kaddr(pfn_t_to_pfn(pfn));
>>
>> pfn_to_kaddr() assumes persistent memory is direct mapped which is not
>> always the case.
>
> Yes.  This is just the default implementation of dax_map_pfn() which works
> for most situations.  We can introduce more complex implementations of
> dax_map_pfn() as necessary.  You make another excellent point for why
> we should store PFNs in the radix tree instead of kaddrs :-)

How much complexity do we want to add in support of an fsync/msync
mechanism that is not the recommended way to use DAX?