Re: [LSF/MM/BPF TOPIC] Swap Abstraction "the pony"

[Jared Hulbert <jaredeh@gmail.com>]

On Tue, Mar 5, 2024 at 1:58 PM Chris Li <chrisl@kernel.org> wrote:
>
> On Tue, Mar 5, 2024 at 1:38 PM Jared Hulbert <jaredeh@gmail.com> wrote:
> >
> > On Mon, Mar 4, 2024 at 11:49 PM Chris Li <chrisl@kernel.org> wrote:
> > >
> > > I have considered that as well, that is further than writing from one
> > > swap device to another. The current swap device currently can't accept
> > > write on non page aligned offset. If we allow byte aligned write out
> > > size, the whole swap entry offset stuff needs some heavy changes.
> > >
> > > If we write out 4K pages, and the compression ratio is lower than 50%,
> > > it means a combination of two compressed pages can't fit into one
> > > page.  Which means some of the page read back will need to overflow
> > > into another page. We kind of need a small file system to keep track
> > > of how the compressed data is stored, because it is not page aligned
> > > size any more.
> > >
> > > We can write out zsmalloc blocks of data as it is, however there is no
> > > guarantee the data in zsmalloc blocks have the same LRU order.
> > >
> > > It makes more sense when writing higher order > 0 swap pages. e.g
> > > writing 64K pages in one buffer, then we can write out compressed data
> > > as page boundary aligned and page sizes, accepting the waste on the
> > > last compressed page, might not fill up the whole page.
> >
> > A swap device not a device, until recently, it was a really bad
> > filesystem with no abstractions between the block device and the
> > filesystem.  Zswap and zram are, in some respects, attempts to make
> > specialized filesystems without any of the advantages of using the vfs
> > tooling.
> >
> > What stops us from using an existing compressing filesystem?
>
> The issue is that the swap has a lot of different usage than a typical
> file system. Please take a look at the current different usage cases
> of swap and their related data structures, in the beginning of this
> email thread.  If you want to use an existing file system, you still
> need to to bridge the gap between swap system and file systems. For
> example, the cgroup information is associated with each swap entry.
>
> You can think of swap as  a special file system that can read and
> write 4K objects by keys.  You can always use file system extend
> attributes to track the additional information associated with each
> swap entry.

Yes.  This is what I was trying to say.  While swap dev pretends to be
just a simple index, your opener for this thread mentions a VFS-like
swap interface.  What exactly is the interface you have in mind?  If
it's VFS-like... how does it differ?

>The end of the day, using the existing file system, the
> per swap entry metadata overhead would  likely be much higher than the
> current swap back end. I understand the current swap back end
> organizes the data around swap offset, that makes swap data spreading
> to many different places. That is one reason people might not like it.
> However, it does have pretty minimal per swap entry memory overheads.
>
> The file system can store their meta data on disk, reducing the in
> memory overhead. That has a price that when you swap in a page, you
> might need to go through a few file system metadata reads before you
> can read in the real swapping data.

When I look at all the things being asked of modern swap backends,
compression, tiering, metadata tracking, usage metrics, caching,
backing storage.  There is a lot of potential for reuse from the
filesystem world.  If we truly have a VFS-like swap interface why not
make it easy to facilitate that reuse?

So of course I don't think we should just take stock btrfs and call it
a swap backing store.  When I asked "Why stops us..." I meant to
discuss it to see how far off the vision is.

So let's consider the points you mentioned.

Metadata overhead:
ZRAM uses 1% of the disksize as metadata storage, you can get to 1% or
less with modern filesystems unmodified (depends on a lot of factors)
From a fundamental architecture standpoint it's not a stretch to think
that a modified filesystem would be meet or beat existing swap engines
on metadata overhead.

Too many disk ops:
This is a solid argument for not using most filesystems today.  But
it's also one that is addressable, modern filesystems have lots of
caching and separation of metadata and data.  No reason a variant
can't be made that will not store metadata to disk.

In the traditional VFS space fragmentation and allocation is the
responsibility of the filesystem, not the pagecache or VFS layer (okay
it gets complicated in the corner cases).  If we call swap backends
the swap filesystems then I don't think it's hard to imagine a
modified (or a new) filesystem could be rather easily adapted to
handle many of the things you're looking for if we made a swapping
VFS-like interface that was a truely a clean subset of the VFS
interface

With a whole family of specialized swap filesystems optimized for
different systems and media types you could do buddy allocating,
larger writes, LRU level group allocations, sub-page allocation,
direct writes, compression, tiering, readahead hints, deduplication,
caching, etc with nearly off the shelf code.  And all this with a free
set of stable APIs, tools, conventions, design patterns, and
abstractions to allow for quick and easy innovation in this space.

And if that seems daunting we can start by making existing swap
backends glue into the new VFS-like interface and punt this for later.
But making clear and clean VFS like interfaces, if done right allows
for a ton of innovation here.

> >
> > Crazy talk here.  What if we handled swap pages like they were mmap'd
> > to a special swap "file(s)"?
>
> That is already the case in the kernel, the swap cache handling is the
> same way of handling file cache with a file offset. Some of them even
> share the same underlying function, for example filemap_get_folio().

Right there is some similarity in the middle.  And yet the way a
swapped page is handled is very different at the "ends"  the PTEs /
fault paths and the way data gets to swap media are totally different.
Those are the parts I was thinking about.  In otherwords, why do a
VFS-like interface, why not use the VFS interface?

I suppose the first level why gets you to something like a circular
reference issue when allocating memory for a VFS ops that could
trigger swapping... but maybe that's addressable.  It gets crazy but I
have a feeling the core issues are not too serious.

> Chris