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

[YoungJun Park]

On Mon, Mar 23, 2026 at 11:32:57AM -0400, Nhat Pham wrote:

> 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@xxxxxxxxx/
> 
> 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
> 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.

This brings back memories -- I'm pretty sure we talked about
exactly this at LPC. Our custom swap device already handles both
zero-filled and same-filled pages on its own, so what we really
wanted was a way to tell the swap layer "just skip the detection
and let it through."
 
I looked at two approaches back then but never submitted either:
 
  - A per-swap_info flag to opt out of zero/same-filled handling.
    But this felt wrong from vswap's perspective -- if even one
    device opts out of the zeromap, the model gets messy.
 
  - Revisiting Usama's patch 2 approach.
    Sounded good in theory, but as you said,
    it's not as simple to verify in practice. And it is more clean design
    swapout time zero check as I see. So,  I gave up on it.
 
Seeing this come up again is actually kind of nice :)
 
One thought -- maybe a compile-time CONFIG or a boot param to
control the scope? e.g. zero-only, same-filled, or disabled.
That way vendors like us just turn it off, and setups like
Kairui's can opt into broader detection. Just an idea though --
open to other approaches if you have something in mind.
 
Thanks,
Youngjun Park