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

[Yosry Ahmed <yosryahmed@google.com>]

On Thu, Feb 22, 2024 at 11:02:24AM -0800, Luis Chamberlain 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

For me, I unfortunately got occuppied with other projects and don't have
the bandwidth to work on it for now :/

I don't want to put anyone on the spot, but I think Nhat may have been
thinking about pursuing a version of this at some point.

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