"Directly mapped persistent memory page cache"

From: Ingo Molnar
Date: Sat May 09 2015 - 04:45:32 EST

* Rik van Riel <riel@xxxxxxxxxx> wrote:

> On 05/08/2015 11:54 AM, Linus Torvalds wrote:
> > On Fri, May 8, 2015 at 7:40 AM, John Stoffel <john@xxxxxxxxxxx> wrote:
> >>
> >> Now go and look at your /home or /data/ or /work areas, where the
> >> endusers are actually keeping their day to day work. Photos, mp3,
> >> design files, source code, object code littered around, etc.
> >
> > However, the big files in that list are almost immaterial from a
> > caching standpoint.
> > The big files in your home directory? Let me make an educated guess.
> > Very few to *none* of them are actually in your page cache right now.
> > And you'd never even care if they ever made it into your page cache
> > *at*all*. Much less whether you could ever cache them using large
> > pages using some very fancy cache.
> However, for persistent memory, all of the files will be "in
> memory".
> Not instantiating the 4kB struct pages for 2MB areas that are not
> currently being accessed with small files may make a difference.
> For dynamically allocated 4kB page structs, we need some way to
> discover where they are. It may make sense, from a simplicity point
> of view, to have one mechanism that works both for pmem and for
> normal system memory.

I don't think we need to or want to allocate page structs dynamically,
which makes the model really simple and robust.

If we 'think big', we can create something very exciting IMHO, that
also gets rid of most of the complications with DIO, DAX, etc:

"Directly mapped pmem integrated into the page cache":

- The pmem filesystem is mapped directly in all cases, it has device
side struct page arrays, and its struct pages are directly in the
page cache, write-through cached. (See further below about how we
can do this.)

Note that this is radically different from the current approach
that tries to use DIO and DAX to provide specialized "direct
access" APIs.

With the 'directly mapped' approach we have numerous advantages:

- no double buffering to main RAM: the device pages represent
file content.

- no bdflush, no VM pressure, no writeback pressure, no
swapping: this is a very simple VM model where the device is
RAM and we don't have much dirty state. The primary kernel
cache is the dcache and the directly mapped page cache, which
is not a writeback cache in this case but essentially a
logical->physical index cache of filesystem indexing

- every binary mmap()ed would be XIP mapped in essence

- every read() would be equivalent a DIO read, without the
complexity of DIO.

- every read() or write() done into a data mmap() area would
allow device-to-device zero copy DMA.

- main RAM caching would still be avilable and would work in
many cases by default: as most apps use file processing
buffers in anonymous memory into which they read() data.

We can achieve this by statically allocating all page structs on the
device, in the following way:

- For every 128MB of pmem data we allocate 2MB of struct-page
descriptors, 64 bytes each, that describes that 128MB data range
in a 4K granular way. We never have to allocate page structs as
they are always there.

- Filesystems don't directly see the preallocated page arrays, they
still get a 'logical block space' presented that to them looks
like a continuous block device (which is 1.5% smaller than the
true size of the device): this allows arbitrary filesystems to be
put into such pmem devices, fsck will just work, etc.

I.e. no special pmem filesystem: the full range of existing block
device based Linux filesystems can be used.

- These page structs are initialized in three layers:

- a single bit at 128MB data granularity: the first struct page
of the 2MB large array (32,768 struct page array members)
represents the initialization state of all of them.

- a single bit at 2MB data granularity: the first struct page
of every 32K array within the 2MB array represents the whole
2MB data area. There are 64 such bits per 2MB array.

- a single bit at 4K data granularity: the whole page array.

A page marked uninitialized at a higher layer means all lower
layer struct pages are in their initial state.

This is a variant of your suggestion: one that keeps everything
2MB aligned, so that a single kernel side 2MB TLB covers a
continuous chunk of the page array. This allows us to create a
linear VMAP physical memory model to simplify index mapping.

- Looking up such a struct page (from a pfn) involves two simple,
easily computable indirections. With locality of access
present, 'hot' struct pages will be in the CPU cache. Them being
64 bytes each will help this. The on-device format is so simple
and so temporary that no fsck is needed for it.

- 2MB mappings, where desired, are 'natural' in such a layout:
everything's 2MB aligned both for kernel and user space use, while
4K granularity is still a first class citizen as well.

- For TB range storage we could make it 1GB granular: We'd allocate
a 1GB array for every 64 GB of data. This would also allow gbpage
TLBs to be taken advantage of: especially on the kernel side
(vmapping the 1GB page array) this might be useful, even if all
actual file usage is 4KB granular. The last block would be allowed
to be smaller than 64GB, but size would still be rounded to 1GB to
keep the mapping simple.

What do you think?


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