Re: Path forward for Virtualized Swap?

[Gregory Price <gourry@gourry.net>]

On Mon, Sep 21, 2026 at 12:01:18PM +0200, Kairui Song wrote:
> > First of all, the traditional swap counter has a very well-defined
> > meaning. It is the size of the memory that, when accessed, requires a
> > page fault. A page fault adds significant latency to memory access
> 
> Yeah I agree on this. Swap just about makes resources not directly
> accessible by the CPU act as RAM, whether that is storage on disk,
> compressed memory, or a network resource, all accessed through a page
> fault. I hope we won't make this fuzzy in the future by introducing
> too many magics.

Hm.  The counters are already fuzzy - the accounting is already doing
two different jobs.

Suppose we want to allow 24GB of logically swapped memory, backed by up
to 8GB of compressed RAM at a 3:1 ratio, but permit only 4GB of physical
swap.

Today we have:
  memory.swap.max  = ?     /* logical memory requiring a fault */
  memory.zswap.max = 8 GB  /* RAM consumed by compressed data */

If memory.swap.max is 4 GB, zswap stops after 4 GB of logical pages,
despite consuming only 1.33GB of RAM.

If memory.swap.max is 24 GB, zswap may reach 8GB of memory consumption,
but the cgroup may also consume up to 24GB of physical swap instead of
the desired 4GB limit.

memory.swap.max is simply overloaded - there is no way for us to express
both limits.

Could we preserve the existing swap semantics and add a physical swap
counter instead?

  memory.swap.max  = 24GB  /* logical swapped memory */
  memory.zswap.max =  8GB  /* compressed RAM limit */
  memory.pswap.max =  4GB  /* physical storage limit */

These limits would be independent and compose naturally.

For a zswap-only workload where we care about RAM consumption but cannot
predict the compression ratio:

  memory.swap.max  = max
  memory.zswap.max = 8GB
  memory.pswap.max = 0

For a workload that performs poorly after more than 7GB of its logical
memory requires swap faults:

  memory.swap.max  = 7GB   /* workload-specific latency/SLO limit */
  memory.zswap.max = 8GB   /* uniform compressed-RAM allowance */
  memory.pswap.max = 0     /* zswap only */

In short:

  memory.swap   = logical swap       - workload/SLO limit
  memory.pswap  = physical swap      - storage limit
  memory.zswap  = compressed memory  - RAM limit

This would preserve the existing memory.swap semantics while allowing
both backing resources to be constrained independently.

~Gregory