RE: what's in nvdimm.git for v4.4?

[Elliott, Robert (Persistent Memory)]

> -----Original Message-----
> From: Linux-nvdimm [mailto:linux-nvdimm-bounces@xxxxxxxxxxxx] On Behalf Of
> Jan Kara
> Sent: Wednesday, October 21, 2015 4:08 AM
...
> On Tue 20-10-15 17:31:18, Dan Williams wrote:
> > On Tue, Oct 20, 2015 at 5:01 PM, Dave Chinner <david@xxxxxxxxxxxxx>
...
> > I'm wondering if we can take a conservative step forward with those
> > patches for 4.4.  if XFS and EXT4 interactions need more time to get
> > worked out, which I believe they do, I can conceive just turning on
> > get_user_pages() support for DAX-mappings of the raw block device.
> > This would be via the new facility I posted yesterday:
> > https://lists.01.org/pipermail/linux-nvdimm/2015-October/002512.html.
> > While not very functional for applications it makes testing base DAX
> > mechanisms straightforward.
> 
> I had a look at the patch and I miss one thing: Why do we need bd_mutex
> to protect faults? I see a comment there:
> 
> /* check that the faulting page hasn't raced with bdev resize */
> 
> Is it really possible that bdev gets shrunk under us? Hum, looking into
> fs/block_dev.c, probably it is. But there are other places - like DIO path
> - assuming that block device mapping cannot just disappear from under us.
> I wonder how that would cope with bdev size change...

DIO_IGNORE_TRUNCATE was added to eliminate an awful lot of CPU
time incrementing and decrementing i_dio_count for direct IO
to block devices, especially at pmem type speeds.  I hope 
that kind of accounting doesn't need to be brought back.

https://lkml.org/lkml/2015/4/3/557
https://lkml.org/lkml/2015/4/15/590

---
Robert Elliott, HPE Persistent Memory

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