Re: "Directly mapped persistent memory page cache"

From: Ingo Molnar
Date: Sun May 10 2015 - 05:46:33 EST

* Dan Williams <dan.j.williams@xxxxxxxxx> 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

> [...] 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

> [...] 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.



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