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

[Chengming Zhou <chengming.zhou@linux.dev>]

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>

> ---
>  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.

> +		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;
>  }
[..]