Re: [RFC] Storing same-filled pages without a zswap_entry

[Johannes Weiner <hannes@cmpxchg.org>]

On Wed, Mar 20, 2024 at 05:07:21PM -0400, Johannes Weiner wrote:
> On Wed, Mar 20, 2024 at 01:49:17PM -0700, Yosry Ahmed wrote:
> > Hey folks,
> > 
> > I was looking at cleaning up the same-filled handling code in zswap,
> > when it hit me that after the xarray conversion, the only member of
> > struct zwap_entry that is relevant to same-filled pages is now the
> > objcg pointer.
> > 
> > The xarray allows a pointer to be tagged by up to two tags (1 and 3),
> > so we can completely avoid allocating a zswap_entry for same-filled
> > pages by storing a tagged objcg pointer directly in the xarray
> > instead.
> > 
> > Basically the xarray would then either have a pointer to struct
> > zswap_entry or struct obj_cgroup, where the latter is tagged as
> > SAME_FILLED_ONE or SAME_FILLED_ZERO.
> > 
> > There are two benefits of this:
> > - Saving some memory (precisely 64 bytes per same-filled entry).
> > - Further separating handling of same-filled pages from compressed
> > pages, which results in some nice cleanups (especially in
> > zswap_store()). It also makes further improvements easier (e.g.
> > skipping limit checking for same-filled entries).
> 
> This sounds interesting.
> 
> Where would you store the byte value it's filled with? Or would you
> limit it to zero-filled only?

The dumb thing about objcg is that for same-filled entries we really
only need it for bumping ZSWPIN. Nothing else. entry->length is 0 for
them, so even though we call the charge function, it doesn't actually
do anything.

Loading them is cheap and doesn't involve decompression. An argument
could be made to exclude them from ZSWPOUT and ZSWPIN entirely.

Or cheat a little and bump ZSWPIN for current->objcg instead -
probably good enough to make excessive thrashing discoverable by the
workload that's directly affected.

Then you could get rid of the objcg pointer and use the xarray slot
for whatever else you'd want.