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

Johannes Weiner
2024-03-28T19:38:55Z

On Mon, Mar 25, 2024 at 11:50:15PM +0000, 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>
> ---
> mm/zswap.c | 137 ++++++++++++++++++++++++++++++-----------------------
> 1 file changed, 78 insertions(+), 59 deletions(-)

Tbh, I think this makes the code worse overall. Instead of increasing
type safety, it actually reduces it, and places that previously dealt
with a struct zswap_entry now deal with a void *.

If we go ahead with this series, I think it makes more sense to either

a) do the explicit subtyping of struct zswap_entry I had proposed, or

b) at least keep the specialness handling of the xarray entry as local
as possible, so that instead of a proliferating API that operates
on void *, you have explicit filtering only where the tree is
accessed, and then work on struct zswap_entry as much as possible.