Re: [LSF/MM/BPF TOPIC] Large folios, swap and fscache

[Chris Li <chrisl@kernel.org>]

On Thu, Feb 22, 2024 at 11:02 AM Luis Chamberlain <mcgrof@kernel.org> wrote:
>
> On Fri, Feb 02, 2024 at 02:29:40PM +0000, Matthew Wilcox wrote:
> > So my modest proposal is that we completely rearchitect how we handle
> > swap.  Instead of putting swp entries in the page tables (and in shmem's
> > case in the page cache), we turn swap into an (object, offset) lookup
> > (just like a filesystem).  That means that each anon_vma becomes its
> > own swap object and each shmem inode becomes its own swap object.
> > The swap system can then borrow techniques from whichever filesystem
> > it likes to do (object, offset, length) -> n x (device, block) mappings.
>
> What happened to Yosry or Chris's last year's pony [0]? In order to try
> to take a stab at this we started with adding large folios to tmpfs,
> which Daniel Gomez has taken on, as its a simple filesystem and with
> large folios can enable us to easily test large folio swap support too.
> Daniel first tried fixing lseek issue with huge pages [1] and on top of
> that he has patches (a new RFC not posted yet) which do add large folios
> support to tmpfs. Hugh has noted the lskeek changes are incorrect and
> suggested instead a fix for the failed tests in fstests. If we get
> agreement on Hugh's approach then we have a step forward with tmpfs and
> later we hope this will make it easier to test swap changes.

Ah, just notice this. I have some pending ideas on how to address
that, I might be the
one that brings up this topic in the discussion David was referring to.

Will reply in his email of this thread.

==== quote ======
On Fri, Feb 2, 2024 at 1:10 AM David Howells <dhowells@redhat.com> wrote:
>
> Hi,
>
> The topic came up in a recent discussion about how to deal with large folios
> when it comes to swap as a swap device is normally considered a simple array
> of PAGE_SIZE-sized elements that can be indexed by a single integer.
>
==== end quote ====

>
> Its probably then a good time to ask, do we have a list of tests for
> swap to ensure we don't break things if we add large folio support?
> We can at least start with a good baseline of tests for that.

That is a very good idea to start with the test. We need all the help
we can get on the testing side.
I know Huge has his own test setup for stressing the swap systems.
Yes, more tests are always better.

Chris

>
> [0] https://lwn.net/Articles/932077/
> [1] https://lkml.kernel.org/r/20240209142901.126894-1-da.gomez@samsung.com
>
>   Luis
>