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

[Barry Song <21cnbao@gmail.com>]

On Sun, Oct 27, 2024 at 3:45 PM Nhat Pham <nphamcs@gmail.com> wrote:
>
> 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?)

agree it is better to have a separate counter for zeromap.
then it raises a question: what is the proper name for it :-)

zeromap_swpin, zeromap_swpout seems too long? and zswpin
and zswpout have been used by zswap

Thanks
barry