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

[Domenico Cerasuolo <cerasuolodomenico@gmail.com>]

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On Tue, Oct 17, 2023 at 7:44 PM Ryan Roberts <ryan.roberts@arm.com> wrote:

> On 19/09/2023 18:14, Nhat Pham 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.
> >
> > Signed-off-by: Domenico Cerasuolo <cerasuolodomenico@gmail.com>
> > Co-developed-by: Nhat Pham <nphamcs@gmail.com>
> > Signed-off-by: Nhat Pham <nphamcs@gmail.com>
> > ---
> >  include/linux/list_lru.h   |  39 +++++++
> >  include/linux/memcontrol.h |   5 +
> >  include/linux/zswap.h      |   9 ++
> >  mm/list_lru.c              |  46 ++++++--
> >  mm/swap_state.c            |  19 ++++
> >  mm/zswap.c                 | 221 +++++++++++++++++++++++++++++--------
> >  6 files changed, 287 insertions(+), 52 deletions(-)
> >
>
> [...]
>
> > @@ -1199,8 +1272,10 @@ bool zswap_store(struct folio *folio)
> >       struct scatterlist input, output;
> >       struct crypto_acomp_ctx *acomp_ctx;
> >       struct obj_cgroup *objcg = NULL;
> > +     struct mem_cgroup *memcg = NULL;
> >       struct zswap_pool *pool;
> >       struct zpool *zpool;
> > +     int lru_alloc_ret;
> >       unsigned int dlen = PAGE_SIZE;
> >       unsigned long handle, value;
> >       char *buf;
> > @@ -1218,14 +1293,15 @@ bool zswap_store(struct folio *folio)
> >       if (!zswap_enabled || !tree)
> >               return false;
> >
> > -     /*
> > -      * XXX: zswap reclaim does not work with cgroups yet. Without a
> > -      * cgroup-aware entry LRU, we will push out entries system-wide
> based on
> > -      * local cgroup limits.
> > -      */
> >       objcg = get_obj_cgroup_from_folio(folio);
> > -     if (objcg && !obj_cgroup_may_zswap(objcg))
> > -             goto reject;
> > +     if (objcg && !obj_cgroup_may_zswap(objcg)) {
> > +             memcg = get_mem_cgroup_from_objcg(objcg);
> > +             if (shrink_memcg(memcg)) {
> > +                     mem_cgroup_put(memcg);
> > +                     goto reject;
> > +             }
> > +             mem_cgroup_put(memcg);
> > +     }
> >
> >       /* reclaim space if needed */
> >       if (zswap_is_full()) {
> > @@ -1240,7 +1316,11 @@ bool zswap_store(struct folio *folio)
> >               else
> >                       zswap_pool_reached_full = false;
> >       }
> > -
> > +     pool = zswap_pool_current_get();
> > +     if (!pool) {
> > +             ret = -EINVAL;
> > +             goto reject;
> > +     }
>
>
> Hi, I'm working to add support for large folios within zswap, and noticed
> this
> piece of code added by this change. I don't see any corresponding put.
> Have I
> missed some detail or is there a bug here?
>
>
> >       /* allocate entry */
> >       entry = zswap_entry_cache_alloc(GFP_KERNEL);
> >       if (!entry) {
> > @@ -1256,6 +1336,7 @@ bool zswap_store(struct folio *folio)
> >                       entry->length = 0;
> >                       entry->value = value;
> >                       atomic_inc(&zswap_same_filled_pages);
> > +                     zswap_pool_put(pool);
>
> I see you put it in this error path, but after that, there is no further
> mention.
>
> >                       goto insert_entry;
> >               }
> >               kunmap_atomic(src);
> > @@ -1264,6 +1345,15 @@ bool zswap_store(struct folio *folio)
> >       if (!zswap_non_same_filled_pages_enabled)
> >               goto freepage;
> >
> > +     if (objcg) {
> > +             memcg = get_mem_cgroup_from_objcg(objcg);
> > +             lru_alloc_ret = memcg_list_lru_alloc(memcg,
> &pool->list_lru, GFP_KERNEL);
> > +             mem_cgroup_put(memcg);
> > +
> > +             if (lru_alloc_ret)
> > +                     goto freepage;
> > +     }
> > +
> >       /* if entry is successfully added, it keeps the reference */
> >       entry->pool = zswap_pool_current_get();
>
> The entry takes it's reference to the pool here.
>
> Thanks,
> Ryan
>

Thanks Ryan, I think you're right. Coincidentally, we're about to send a new
version of the series, and will make sure to address this too.

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