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

[Johannes Weiner <hannes@cmpxchg.org>]

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.

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.