Re: "Directly mapped persistent memory page cache"

[Dan Williams]

On Sun, May 10, 2015 at 2:46 AM, Ingo Molnar <mingo@xxxxxxxxxx> wrote:
>
> * 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
> opt-in.
>

Hmm, my only hesitation is that the raw size of a pmem device is
visible outside of Linux (UEFI/BIOS other OSes, etc).  What about a
simple layered block-device that fronts a raw pmem device?  It can
store a small superblock signature and then reserve / init the struct
page space.  This is where we can use the __pfn_t flags that Linus
suggested.  Whereas the raw device ->direct_access() returns __pfn_t
values with a 'PFN_DEV' flag indicating originating from
device-memory, a ->direct_access() on this struct-page-provider-device
yields __pfn_t's with 'PFN_DEV | PFN_MAPPED' indicating that
__pfn_t_to_page() can attempt to lookup the page in a device-specific
manner (similar to how kmap_atomic_pfn_t is implemented in the
'evacuate struct page from the block layer' patch set).
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