Re: [PATCH v5 00/21] Virtual Swap Space

[Nhat Pham <nphamcs@gmail.com>]

On Mon, Mar 23, 2026 at 12:41 PM Kairui Song <ryncsn@gmail.com> wrote:
>
> On Mon, Mar 23, 2026 at 11:33 PM Nhat Pham <nphamcs@gmail.com> wrote:
> >
> > On Mon, Mar 23, 2026 at 6:09 AM Kairui Song <ryncsn@gmail.com> wrote:
> > >
> > > On Sat, Mar 21, 2026 at 3:29 AM Nhat Pham <nphamcs@gmail.com> wrote:
> > > > This patch series is based on 6.19. There are a couple more
> > > > swap-related changes in mainline that I would need to coordinate
> > > > with, but I still want to send this out as an update for the
> > > > regressions reported by Kairui Song in [15]. It's probably easier
> > > > to just build this thing rather than dig through that series of
> > > > emails to get the fix patch :)
> > > >
> > > > Changelog:
> > > > * v4 -> v5:
> > > >     * Fix a deadlock in memcg1_swapout (reported by syzbot [16]).
> > > >     * Replace VM_WARN_ON(!spin_is_locked()) with lockdep_assert_held(),
> > > >       and use guard(rcu) in vswap_cpu_dead
> > > >       (reported by Peter Zijlstra [17]).
> > > > * v3 -> v4:
> > > >     * Fix poor swap free batching behavior to alleviate a regression
> > > >       (reported by Kairui Song).
> > >
> >
> > Hi Kairui! Thanks a lot for the testing big boss :) I will focus on
> > the regression in this patch series - we can talk more about
> > directions in another thread :)
>
> Hi Nhat,
>
> > Interesting. Normally "lots of zero-filled page" is a very beneficial
> > case for vswap. You don't need a swapfile, or any zram/zswap metadata
> > overhead - it's a native swap backend. If production workload has this
> > many zero-filled pages, I think the numbers of vswap would be much
> > less alarming - perhaps even matching memory overhead because you
> > don't need to maintain a zram entry metadata (it's at least 2 words
> > per zram entry right?), while there's no reverse map overhead induced
> > (so it's 24 bytes on both side), and no need to do zram-side locking
> > :)
> >
> > So I was surprised to see that it's not working out very well here. I
> > checked the implementation of memhog - let me know if this is wrong
> > place to look:
> >
> > https://man7.org/linux/man-pages/man8/memhog.8.html
> > https://github.com/numactl/numactl/blob/master/memhog.c#L52
> >
> > I think this is what happened here: memhog was populating the memory
> > 0xff, which triggers the full overhead of a swapfile-backed swap entry
> > because even though it's "same-filled" it's not zero-filled! I was
> > following Usama's observation - "less than 1% of the same-filled pages
> > were non-zero" - and so I only handled the zero-filled case here:
> >
> > https://lore.kernel.org/all/20240530102126.357438-1-usamaarif642@gmail.com/
> >
> > This sounds a bit artificial IMHO - as Usama pointed out above, I
> > think most samefilled pages are zero pages, in real production
> > workloads. However, if you think there are real use cases with a lot
>
> I vaguely remember some workloads like Java or some JS engine
> initialize their heap with fixed value, same fill might not be that
> common but not a rare thing, it strongly depends on the workload.

To a non-zero value? ISTR it was initialized to zero, but if I was
wrong then yeah it should just be a small simple patch.

>
> > of non-zero samefilled pages, please let me know I can fix this real
> > quick. We can support this in vswap with zero extra metadata overhead
> > - change the VSWAP_ZERO swap entry type to VSWAP_SAME_FILLED, then use
> > the backend field to store that value. I can send you a patch if
> > you're interested.
>
> Actually I don't think that's the main problem. For example, I just
> wrote a few lines C bench program to zerofill ~50G of memory
> and swapout sequentially:
>
> Before:
> Swapout: 4415467us
> Swapin: 49573297us
>
> After:
> Swapout: 4955874us
> Swapin: 56223658us
>
> And vmstat:
> cat /proc/vmstat | grep zero
> thp_zero_page_alloc 0
> thp_zero_page_alloc_failed 0
> swpin_zero 12239329
> swpout_zero 21516634
>
> There are all zero filled pages, but still slower. And what's more, a
> more critical issue, I just found the cgroup and global swap usage
> accounting are both somehow broken for zero page swap,
> maybe because you skipped some allocation? Users can
> no longer see how many pages are swapped out. I don't think you can
> break that, that's one major reason why we use a zero entry instead of
> mapping to a zero readonly page. If that is acceptable, we can have
> a very nice optimization right away with current swap.

No, that was intentional :) I probably should have documented this
better - but we're only charging towards swap usage (cgroup and system
wide) on memory. There was a whole patch that did that in the series
:)

I can add new counters to differentiate these cases, but it makes no
sense to me to charge towards swap usage for non-swapfile backend
(namely, zswap and zero swap pages). You are not actually occupying
the limited swapfile slots, but instead occupy a dynamic, vast virtual
swap space only (and memory in the case of zswap - this is actually an
argument against zram which does not do any cgroup accounting, but
that's another story for another day). I don't see a point in swap
charging here. It's the whole point of decoupling the backends - these
are not the same resource domains.

And if you follow Usama's work above, we actually were trying to
figure out a way to map it to a zero readonly page. That was Usama's
v2 of the patch series IIRC - but there was a bug. I think it was a
potential race between the reclaimer's rmap walk to unmap the page
from PTEs pointing to the page, and concurrent modifiers to the page?
We couldn't fix the race in a way that does not induce more overhead
than it's worth. But had that work we would also not do any swap
charging :)

BTW, if you can figure that part out, please let us know. We actually
quite like that idea - we just never managed to make it work (and we
have a bunch more urgent tasks).

>
> That's still just an example. bypassing the accounting and still
> slower is not a good sign. We should focus on the generic
> performance and design.

I will dig into the remaining regression :) Thanks for the report.

>
> Yet this is just another new found issue, there are many other parts
> like the folio swap allocation may still occur even if a lower device
> can no longer accept more whole folios, which I'm currently
> unsure how it will affect swap.



>
> > 1. Regarding pmem backend - I'm not sure if I can get my hands on one
> > of these, but if you think SSD has the same characteristics maybe I
> > can give that a try? The problem with SSD is for some reason variance
> > tends to be pretty high, between iterations yes, but especially across
> > reboots. Or maybe zram?
>
> Yeah, ZRAM has a very similar number for some cases, but storage is
> getting faster and faster and swap occurs through high speed networks
> too. We definitely shouldn't ignore that.

I can also simulate it using tmpfs as a swap backend (although it
might not work for certain benchmarks, like your usemem benchmark in
which we allocate more memory than the host physical memory).

>
> > 2. What about the other numbers below? Are they also on pmem? FTR I
> > was running most of my benchmarks on zswap, except for one kernel
> > build benchmark on SSD.
> >
> > 3. Any other backends and setup you're interested in?
> >
> > BTW, sounds like you have a great benchmark suite - is it open source
> > somewhere? If not, can you share it with us :) Vswap aside, I think
> > this would be a good suite to run all swap related changes for every
> > swap contributor.
>
> I can try to post that somewhere, really nothing fancy just some
> wrapper to make use of systemd for reboot and auto test. But all test
> steps I mentioned before are already posted and publically available.

Okay, thanks, Kairui!