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

From: Andrew Morton
Date: Tue May 30 2023 - 17:16:18 EST

On Tue, 30 May 2023 21:02:51 +0000 Yosry Ahmed <yosryahmed@xxxxxxxxxx> 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

> 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.
> + 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?