Re: Path forward for Virtualized Swap?

[Johannes Weiner <hannes@cmpxchg.org>]

On Sat, Sep 12, 2026 at 05:00:42PM +0800, Kairui Song wrote:
> On Thu, Sep 10, 2026 at 12:42 AM Nhat Pham <nphamcs@gmail.com> wrote:
> >
> > On Tue, Sep 8, 2026 at 11:30 AM Johannes Weiner <hannes@cmpxchg.org> wrote:
> > >
> > > On Mon, Sep 07, 2026 at 01:51:31PM +0800, Kairui Song wrote:
> > > > Where I've ended up is that unbounded growth is a real concern. On a
> > > > host with no memcg limit (root cgroup, and most desktop and embedded
> > > > setups), an unlimited pool means usage can keep growing, with no
> > > > admin visible ceiling at all. I'm not attached to xswap's percent of RAM
> > > > knob specifically, but I do think some kind of bound makes sense.
> > >
> > > Swap space is just process virtual address space, no?
> > >
> > > Swap entries already have one or more page table entries pointing to
> > > them, which in turn are managed by trees of vm_area_structs. That
> > > means rlimits apply, overcommit protection applies, and OOM killer
> > > attribution works as well (oom_badness()).
> >
> > I tested this theory. I spinned up a process, and let it spam 0-filled
> > memory + swap these pages out continually.
> >
> > As you predicted, oom-killer picked it up eventually. The host was
> > (and is) intact otherwise :)
> 
> Good to know the kill path works. But I think the accounting side cuts the
> other way? Won't that conversely underestimate the host's ability to
> handle memory alloc? Not to mention a lot of application are
> swap  space aware, some built in logics like e.g. with vm_enough_memory:
> 
> On a 1G machine with vswap enabled:
> [    0.241906] vswap: created virtual swap device (2199023255040 pages)
> 
> a 2G sparse anonymous allocation fails:
> [   46.044002] __vm_enough_memory: pid: 1129, comm: search_agent,
> bytes: 2147483648 not enough memory for the allocation.
> 
> The default "Heuristic overcommit handling" policy is meant to handle
> seriously wild allocation (as documented, and it's named as
> OVERCOMMIT_GUESS).
> totalram_pages() + total_swap_pages
> 
> Is the limit, the heuristic exists only to make "a seriously wild
> allocation fail" (as documented). But on a vswap-only machine,
> reasonable allocations fail. Common swap devices, zram, or xswap don't
> have this problem. One could argue that the size there is just an
> optimistic guess, the compression ratio is not controllable. But that
> heuristic is a guess by design, and a plausible number serves it fine.
> "Unlimited" seems break that.

That cuts both ways, though. If the configured compression space is
large and compression ratio is poor, it lets through allocations that
can be considered "seriously wild" for this machine.

It's a filter. I wouldn't say that letting things through is evidence
that it's working.

Since compression space is backed by memory, it still makes sense to
me to reference this heuristic to RAM. Cut it off at 2xRAM or 3xRAM
tops if compression space is available.

That all being said, I've laid out implications and concerns around
limiting compression space in this thread, too. Collecting pros and
cons is fine, but if you bring up cons for the unlimited case, it's
fair to ask that you engage with cons brought up on the limited case,
too, so that we can weigh the tradeoffs.