Re: [PATCH v2 2/2] mm/zswap: Support batch writeback in shrink_memcg()

[Yosry Ahmed]

On Fri, Jul 24, 2026 at 11:37 AM Nhat Pham <nphamcs@xxxxxxxxx> wrote:
>
> On Fri, Jul 24, 2026 at 10:57 AM Yosry Ahmed <yosry@xxxxxxxxxx> wrote:
> >
> > The numbers generally look good, the store rejection rate is
> > decreasing and we are naturally doing more writeback which makes
> > sense. The store rejection rate actually increases when the batch size
> > increases to 64, maybe aggressive writeback is more likely to fail and
> > cause a store rejection, so that also makes sense.
> >
> > The only part I can't immediately reason about is pswpin. How are we
> > reading from physical swap more than we ever wrote to it?
>
> The same page can be swapped out once, and swap in multiple times :)
>
> Unlike zswap, physical swap in is not necessarily exclusive. You load
> the page in memory, but if the swapfile is not full, etc. etc. you
> don't invalidate the copy of the data on the swapfile. At reclaim
> time, we notice the page is not dirty, so we just skip (z)swapout.

Oh right, I forgot about that, thanks.

I still don't fully understand why pswpin is significantly increased
with batching here. I would hope that with less LRU inversion we end
up with less disk swapin. Probably the test access patterns are just
too random compared to real workloads?