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

[Yosry Ahmed <yosryahmed@google.com>]

On Thu, Mar 21, 2024 at 11:40:32AM +0800, Chengming Zhou wrote:
> On 2024/3/21 05:31, Yosry Ahmed wrote:
> > On Wed, Mar 20, 2024 at 2:19 PM Johannes Weiner <hannes@cmpxchg.org> wrote:
> >>
> >> 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).
> 
> I also think this is a good idea. :) Which could simplify the code too.
> 
> >>>
> >>> 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.
> > 
> > Yeah it's only useful for the stats. Using current->objcg would work,
> > and should be ultimately pointing to the same memcg in *most* cases, I
> 
> In some cases where the current objcg is not "correct", the testcases in
> test_zswap.c may break? Maybe we can use swap_cgroup info to charge the
> stats to the correct memcg? Not sure if this is feasible.

For cgroup v1, swap_cgroup will be cleared from
mem_cgroup_swapin_uncharge_swap() before the zswap load.

I think the current objcg will remain correct as long as swapin happens
from the same memcg as swapout (or if swapin happens from the parent
memcg and the swapout memcg was offlined).