Re: "Directly mapped persistent memory page cache"

[Ingo Molnar <mingo@kernel.org>]

* Dan Williams <dan.j.williams@intel.com> wrote:

> > "Directly mapped pmem integrated into the page cache":
> > ------------------------------------------------------

> Nice, I think it makes sense as an area that gets reserved at file 
> system creation time.  You are not proposing that this gets 
> automatically reserved at the device level, right? [...]

Well, it's most practical if the device does it automatically (the 
layout is determined prior filesystem creation), and the filesystem 
does not necessarily have to be aware of it - but obviously as a user 
opt-in.

> [...] For the use cases of persistent memory in the absence of a 
> file system (in-kernel managed hierarchical storage driver, or 
> mmap() the pmem block device directly) there's still a need for 
> pfn-based DAX. [...]

Yes, but knowing that there's a sane model we can take a hard stance 
against pfn proliferation and say: 'we let pfns this far and no 
farther'.

> [...]  In other words, simple usages skip the overhead.  They assume 
> pfns only, discover those pmem pfns via ->direct_access(), and 
> arrange for exclusive block device ownership.  Everything else that 
> requires struct page in turn requires a file system to have opt-ed 
> into this reservation and provide pmem-struct-page infrastructure, 
> pmem-aware-DIO etc.

Yes.

Thanks,

	Ingo