Re: [PATCH v2 1/2] zswap: make shrinking memcg-aware

[Yosry Ahmed <yosryahmed@google.com>]

On Tue, Sep 26, 2023 at 11:24 AM Johannes Weiner <hannes@cmpxchg.org> wrote:
>
> On Mon, Sep 25, 2023 at 01:17:04PM -0700, Yosry Ahmed wrote:
> > +Chris Li
> >
> > On Tue, Sep 19, 2023 at 10:14 AM Nhat Pham <nphamcs@gmail.com> wrote:
> > >
> > > From: Domenico Cerasuolo <cerasuolodomenico@gmail.com>
> > >
> > > Currently, we only have a single global LRU for zswap. This makes it
> > > impossible to perform worload-specific shrinking - an memcg cannot
> > > determine which pages in the pool it owns, and often ends up writing
> > > pages from other memcgs. This issue has been previously observed in
> > > practice and mitigated by simply disabling memcg-initiated shrinking:
> > >
> > > https://lore.kernel.org/all/20230530232435.3097106-1-nphamcs@gmail.com/T/#u
> > >
> > > This patch fully resolves the issue by replacing the global zswap LRU
> > > with memcg- and NUMA-specific LRUs, and modify the reclaim logic:
> > >
> > > a) When a store attempt hits an memcg limit, it now triggers a
> > >    synchronous reclaim attempt that, if successful, allows the new
> > >    hotter page to be accepted by zswap.
> > > b) If the store attempt instead hits the global zswap limit, it will
> > >    trigger an asynchronous reclaim attempt, in which an memcg is
> > >    selected for reclaim in a round-robin-like fashion.
> >
> > Hey Nhat,
> >
> > I didn't take a very close look as I am currently swamped, but going
> > through the patch I have some comments/questions below.
> >
> > I am not very familiar with list_lru, but it seems like the existing
> > API derives the node and memcg from the list item itself. Seems like
> > we can avoid a lot of changes if we allocate struct zswap_entry from
> > the same node as the page, and account it to the same memcg. Would
> > this be too much of a change or too strong of a restriction? It's a
> > slab allocation and we will free memory on that node/memcg right
> > after.
>
> My 2c, but I kind of hate that assumption made by list_lru.
>
> We ran into problems with it with the THP shrinker as well. That one
> strings up 'struct page', and virt_to_page(page) results in really fun
> to debug issues.
>
> IMO it would be less error prone to have memcg and nid as part of the
> regular list_lru_add() function signature. And then have an explicit
> list_lru_add_obj() that does a documented memcg lookup.

I also didn't like/understand that assumption, but again I don't have
enough familiarity with the code to judge, and I don't know why it was
done that way. Adding memcg and nid as arguments to the standard
list_lru API makes the pill easier to swallow. In any case, this
should be done in a separate patch to make the diff here more focused
on zswap changes.

>
> Because of the overhead, we've been selective about the memory we
> charge. I'd hesitate to do it just to work around list_lru.

On the other hand I am worried about the continuous growth of struct
zswap_entry. It's now at ~10 words on 64-bit? That's ~2% of the size
of the page getting compressed if I am not mistaken. So I am skeptical
about storing the nid there.

A middle ground would be allocating struct zswap_entry on the correct
node without charging it. We don't need to store the nid and we don't
need to charge struct zswap_entry. It doesn't get rid of
virt_to_page() though.