Re: [PATCH] mm:zswap: fix zswap entry reclamation failure in two scenarios

[Yosry Ahmed <yosryahmed@google.com>]

On Thu, Nov 16, 2023 at 12:30 PM Chris Li <chriscli@google.com> wrote:
>
> On Thu, Nov 16, 2023 at 12:19 PM Yosry Ahmed <yosryahmed@google.com> wrote:
> >
> > Not bypassing the swap slot cache, just make the callbacks to
> > invalidate the zswap entry, do memg uncharging, etc when the slot is
> > no longer used and is entering the swap slot cache (i.e. when
> > free_swap_slot() is called), instead of when draining the swap slot
> > cache (i.e. when swap_range_free() is called). For all parts of MM
> > outside of swap, the swap entry is freed when free_swap_slot() is
> > called. We don't free it immediately because of caching, but this
> > should be transparent to other parts of MM (e.g. zswap, memcg, etc).
>
> That will cancel the batching effect on the swap slot free, making the
> common case for  swapping  faults take longer to complete, righ?
> If I recall correctly, the uncharge is the expensive part of the swap
> slot free operation.
> I just want to figure out what we are trading off against. This is not
> one side wins all situations.

Interesting. Maybe we can just move the zswap_invalidate() call to
save memory early, and leave the memcg uncharge call to be batched?
IIUC we already do some version of this internally at Google.

>
>
> Chris