On Thu, 19 Mar 2015 17:54:15 +0200 Boaz Harrosh [off-list ref] wrote:

On 03/19/2015 03:43 PM, Matthew Wilcox wrote:
<>

quoted

Dan missed "Support O_DIRECT to a mapped DAX file".  More generally, if we
want to be able to do any kind of I/O directly to persistent memory,
and I think we do, we need to do one of:

1. Construct struct pages for persistent memory
1a. Permanently
1b. While the pages are under I/O
2. Teach the I/O layers to deal in PFNs instead of struct pages
3. Replace struct page with some other structure that can represent both
   DRAM and PMEM

I'm personally a fan of #3, and I was looking at the scatterlist as
my preferred data structure.  I now believe the scatterlist as it is
currently defined isn't sufficient, so we probably end up needing a new
data structure.  I think Dan's preferred method of replacing struct
pages with PFNs is actually less instrusive, but doesn't give us as
much advantage (an entirely new data structure would let us move to an
extent based system at the same time, instead of sticking with an array
of pages).  Clearly Boaz prefers 1a, which works well enough for the
8GB NV-DIMMs, but not well enough for the 400GB NV-DIMMs.

What's your preference?  I guess option 0 is "force all I/O to go
through the page cache and then get copied", but that feels like a nasty
performance hit.

Thanks Matthew, you have summarized it perfectly.

I think #1b might have merit, as well.

It would be interesting to see what a 1b implementation looks like and
how it performs.  We already allocate a bunch of temporary things to
support in-flight IO (bio, request) and allocating pageframes on the
same basis seems a fairly logical fit.

It is all a bit of a stopgap, designed to shoehorn
direct-io-to-dax-mapped-memory into the existing world.  Longer term
I'd expect us to move to something more powerful, but it's unclear what
that will be at this time, so a stopgap isn't too bad?


This is all contingent upon the prevalence of machines which have vast
amounts of nv memory and relatively small amounts of regular memory. 
How confident are we that this really is the future?