Re: [PATCH RFC] mm: count zeromap read and set for swapout and swapin

[Nhat Pham <nphamcs@gmail.com>]

On Sat, Oct 26, 2024 at 6:20 PM Barry Song <21cnbao@gmail.com> wrote:
>
> From: Barry Song <v-songbaohua@oppo.com>
>
> When the proportion of folios from the zero map is small, missing their
> accounting may not significantly impact profiling. However, it’s easy
> to construct a scenario where this becomes an issue—for example,
> allocating 1 GB of memory, writing zeros from userspace, followed by
> MADV_PAGEOUT, and then swapping it back in. In this case, the swap-out
> and swap-in counts seem to vanish into a black hole, potentially
> causing semantic ambiguity.

I agree. It also makes developing around this area more challenging.
I'm working on the swap abstraction, and sometimes I can't tell if I
screwed up somewhere, or if a proportion of these allocated entries go
towards this optimization...

Thanks for taking a stab at fixing this, Barry!

>
> We have two ways to address this:
>
> 1. Add a separate counter specifically for the zero map.
> 2. Continue using the current accounting, treating the zero map like
> a normal backend. (This aligns with the current behavior of zRAM
> when supporting same-page fills at the device level.)

Hmm, my understanding of the pswpout/pswpin counters is that they only
apply to IO done directly to the backend device, no? That's why we
have a separate set of counters for zswap, and do not count them
towards pswp(in|out).

For users who have swap files on physical disks, the performance
difference between reading directly from the swapfile and going
through these optimizations could be really large. I think it makes
sense to have a separate set of counters for zero-mapped pages
(ideally, both at the host level and at the cgroup level?)