Re: [RFC PATCH 7/9] mm: zswap: store zero-filled pages without a zswap_entry

Yosry Ahmed
2024-03-28T18:46:05Z

On Thu, Mar 28, 2024 at 1:12 AM Chengming Zhou <chengming.zhou@linux.dev> wrote:
>
> On 2024/3/26 07:50, Yosry Ahmed wrote:
> > After the rbtree to xarray conversion, and dropping zswap_entry.refcount
> > and zswap_entry.value, the only members of zswap_entry utilized by
> > zero-filled pages are zswap_entry.length (always 0) and
> > zswap_entry.objcg. Store the objcg pointer directly in the xarray as a
> > tagged pointer and avoid allocating a zswap_entry completely for
> > zero-filled pages.
> >
> > This simplifies the code as we no longer need to special case
> > zero-length cases. We are also able to further separate the zero-filled
> > pages handling logic and completely isolate them within store/load
> > helpers. Handling tagged xarray pointers is handled in these two
> > helpers, as well as the newly introduced helper for freeing tree
> > elements, zswap_tree_free_element().
> >
> > There is also a small performance improvement observed over 50 runs of
> > kernel build test (kernbench) comparing the mean build time on a skylake
> > machine when building the kernel in a cgroup v1 container with a 3G
> > limit. This is on top of the improvement from dropping support for
> > non-zero same-filled pages:
> >
> > base patched % diff
> > real 69.915 69.757 -0.229%
> > user 2956.147 2955.244 -0.031%
> > sys 2594.718 2575.747 -0.731%
> >
> > This probably comes from avoiding the zswap_entry allocation and
> > cleanup/freeing for zero-filled pages. Note that the percentage of
> > zero-filled pages during this test was only around 1.5% on average.
> > Practical workloads could have a larger proportion of such pages (e.g.
> > Johannes observed around 10% [1]), so the performance improvement should
> > be larger.
> >
> > This change also saves a small amount of memory due to less allocated
> > zswap_entry's. In the kernel build test above, we save around 2M of
> > slab usage when we swap out 3G to zswap.
> >
> > [1]https://lore.kernel.org/linux-mm/20240320210716.GH294822@cmpxchg.org/
> >
> > Signed-off-by: Yosry Ahmed <yosryahmed@google.com>
>
> The code looks good, just one comment below.
>
> Reviewed-by: Chengming Zhou <chengming.zhou@linux.dev>

Thanks!

>
> > ---
> > mm/zswap.c | 137 ++++++++++++++++++++++++++++++-----------------------
> > 1 file changed, 78 insertions(+), 59 deletions(-)
> >
> > diff --git a/mm/zswap.c b/mm/zswap.c
> > index 413d9242cf500..efc323bab2f22 100644
> > --- a/mm/zswap.c
> > +++ b/mm/zswap.c
> > @@ -183,12 +183,11 @@ static struct shrinker *zswap_shrinker;
> > * struct zswap_entry
> > *
> [..]
> >
> > @@ -1531,26 +1552,27 @@ bool zswap_load(struct folio *folio)
> > struct page *page = &folio->page;
> > struct xarray *tree = swap_zswap_tree(swp);
> > struct zswap_entry *entry;
> > + struct obj_cgroup *objcg;
> > + void *elem;
> >
> > VM_WARN_ON_ONCE(!folio_test_locked(folio));
> >
> > - entry = xa_erase(tree, offset);
> > - if (!entry)
> > + elem = xa_erase(tree, offset);
> > + if (!elem)
> > return false;
> >
> > - if (entry->length)
> > + if (!zswap_load_zero_filled(elem, page, &objcg)) {
> > + entry = elem;
>
> nit: entry seems no use anymore.

I left it here on purpose to avoid casting elem in the next two lines,
it is just more aesthetic.

>
> > + objcg = entry->objcg;
> > zswap_decompress(entry, page);
> > - else
> > - clear_highpage(page);
> > + }
> >
> > count_vm_event(ZSWPIN);
> > - if (entry->objcg)
> > - count_objcg_event(entry->objcg, ZSWPIN);
> > -
> > - zswap_entry_free(entry);
> > + if (objcg)
> > + count_objcg_event(objcg, ZSWPIN);
> >
> > + zswap_tree_free_element(elem);
> > folio_mark_dirty(folio);
> > -
> > return true;
> > }
> [..]