Re: [PATCH] mm: zswap: support exclusive loads

[Andrew Morton <akpm@linux-foundation.org>]

On Tue, 30 May 2023 21:02:51 +0000 Yosry Ahmed <yosryahmed@google.com> wrote:

> Commit 71024cb4a0bf ("frontswap: remove frontswap_tmem_exclusive_gets")
> removed support for exclusive loads from frontswap as it was not used.
> 
> Bring back exclusive loads support to frontswap by adding an
> exclusive_loads argument to frontswap_ops. Add support for exclusive
> loads to zswap behind CONFIG_ZSWAP_EXCLUSIVE_LOADS.

Why is this Kconfigurable?  Why not just enable the feature for all
builds?

> Refactor zswap entry invalidation in zswap_frontswap_invalidate_page()
> into zswap_invalidate_entry() to reuse it in zswap_frontswap_load().
> 
> With exclusive loads, we avoid having two copies of the same page in
> memory (compressed & uncompressed) after faulting it in from zswap. On
> the other hand, if the page is to be reclaimed again without being
> dirtied, it will be re-compressed. Compression is not usually slow, and
> a page that was just faulted in is less likely to be reclaimed again
> soon.
> 
> --- a/mm/Kconfig
> +++ b/mm/Kconfig
> @@ -46,6 +46,19 @@ config ZSWAP_DEFAULT_ON
>  	  The selection made here can be overridden by using the kernel
>  	  command line 'zswap.enabled=' option.
>  
> +config ZSWAP_EXCLUSIVE_LOADS
> +	bool "Invalidate zswap entries when pages are loaded"
> +	depends on ZSWAP
> +	help
> +	  If selected, when a page is loaded from zswap, the zswap entry is
> +	  invalidated at once, as opposed to leaving it in zswap until the
> +	  swap entry is freed.
> +
> +	  This avoids having two copies of the same page in memory
> +	  (compressed and uncompressed) after faulting in a page from zswap.
> +	  The cost is that if the page was never dirtied and needs to be
> +	  swapped out again, it will be re-compressed.

So it's a speed-vs-space tradeoff?  I'm not sure how users are to
decide whether they want this.  Did we help them as much as possible?