Re: "Directly mapped persistent memory page cache"

[Dave Chinner]

On Sat, May 09, 2015 at 10:45:10AM +0200, Ingo Molnar wrote:
> 
> * 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

But, OTOH, no encryption, no compression, no
mirroring/redundancy/repair, etc.  i.e. it's a model where it is
impossible to do data transformations in the IO path....

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

Sure, it is replaced with the complexity of the buffered read path.
Swings and roundabouts.

>        - 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.

Who allocates them, when do they get allocated, what happens when
they get corrupted?

>   - 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.

Again, what happens when the page arrays get corrupted? You can't
just reboot to make the corruption go away.

i.e. what's the architecture of the supporting userspace utilities
that are needed to manage this persistent page array area?

>     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.

Why wouldn't you just initialise them for the whole device in one
go? If they are transparent to the filesystem address space, then
you have to reserve space for the entire pmem range up front, so
why wouldn't you just initialise them when you reserve the space?

>     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.

What is doing this aligned allocation of the persistent memory
extents? The filesystem, right?

All this talk about page arrays and aligned allocation of pages
for mapping as large pages has to come from the filesystem
allocating large aligned extents. IOWs, the only way we can get
large page mappings in the VM for persistent memory is if the
filesystem managing the persistent memory /does the right thing/.

And, of course, different platforms have different page sizes, so
designing page array structures to be optimal for x86-64 is just a
wee bit premature.

What we need to do is work out how we are going to tell the
filesystem that is managing the persistent memory what the alignment
constraints it needs to work under are.

>   - 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 

A properly designed extent allocator will understand this second
level of alignment.
					VM		    FS
Minimum unit of allocation:	PAGE_SIZE		block size
First unit of alignment:	Large Page Size		stripe unit
Second unit of alignment:	Giant Page Size		stripe width

i.e. this is the information the pmem device needs to feed
the filesystem mkfs program to start it down the correct path.

Next, you need a hint for each file to tell the filesystem what
alignment it should try to allocate with. XFS has extent size hints
for this, and for a 4k page/block size this allows up to 4GB hints
to be set. XFs allocates these as unwritten extents, so if the VM
can only map it as PAGE_SIZE mappings, then everything will still
just work - the dirtied pages will get converted to written, and
everything else will appear as zeros because they remain unwritten.

Map the page as a single 2MB chunk, and then the fs will have to
zero the entire chunk on the first write page fault so it can mark
the entire extent as written data.

IOWs the initialisation state of the struct pages is actually a
property of the filesystem space usage, not a property of the
virtual mappings that are currently active. If the space is in use,
then then the struct pages must be initialised, if the pages are
free space then we don't care what their contents are as nobody can
be accessing them.  Further, we cannot validate that the page array
structures are valid in isolation (we must be able to independently
validate them if they are persistent) and hence  we need to know
whether the pages are referenced by the filesystem or not to
determine whether their state is correct. 

Which comes back to my original question: if the struct page arrays
are outside the visibility of the filesystem, how do we manage them
in a safe and consistent manner? How do we verify they areD correct
coherent with the filesystem using the device when the filesystem
knows nothing about page mapping space, and the page mapping space
knowns nothing about the contents of the pmem device? Indeed, how do we
do transactionally safe updates to thea page arrays to mark them
initialised so that they are atomic w.r.t. the associated filesystem
free space state changes? And dare I say "truncate"?

Cheers,

Dave.
-- 
Dave Chinner
david@xxxxxxxxxxxxx
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