linux-kernel - Re: [PATCH v8] zswap: replace RB tree with xarray

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Date: Mon, 25 Mar 2024 13:35:46 -0700
From: Andrew Morton <akpm@...ux-foundation.org>
To: Chris Li <chrisl@...nel.org>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org, Yosry Ahmed
 <yosryahmed@...gle.com>, Nhat Pham <nphamcs@...il.com>, Johannes Weiner
 <hannes@...xchg.org>, "Matthew Wilcox (Oracle)" <willy@...radead.org>,
 Chengming Zhou <zhouchengming@...edance.com>, Barry Song
 <v-songbaohua@...o.com>, Johannes Weiner <hannes@...xchg.org>
Subject: Re: [PATCH v8] zswap: replace RB tree with xarray

On Wed, 20 Mar 2024 12:31:38 -0700 Chris Li <chrisl@...nel.org> wrote:

> Very deep RB tree requires rebalance at times. That
> contributes to the zswap fault latencies. Xarray does not
> need to perform tree rebalance. Replacing RB tree to xarray
> can have some small performance gain.
> 
> One small difference is that xarray insert might fail with
> ENOMEM, while RB tree insert does not allocate additional
> memory.
> 
> The zswap_entry size will reduce a bit due to removing the
> RB node, which has two pointers and a color field. Xarray
> store the pointer in the xarray tree rather than the
> zswap_entry. Every entry has one pointer from the xarray
> tree. Overall, switching to xarray should save some memory,
> if the swap entries are densely packed.
> 
> Notice the zswap_rb_search and zswap_rb_insert always
> followed by zswap_rb_erase. Use xa_erase and xa_store
> directly. That saves one tree lookup as well.
> 
> Remove zswap_invalidate_entry due to no need to call
> zswap_rb_erase any more. Use zswap_free_entry instead.
> 
> The "struct zswap_tree" has been replaced by "struct xarray".
> The tree spin lock has transferred to the xarray lock.
> 
> Run the kernel build testing 10 times for each version, averages:
> (memory.max=2GB, zswap shrinker and writeback enabled,
> one 50GB swapfile, 24 HT core, 32 jobs)
> 

So this conflits with Johannes's "mm: zswap: fix data loss on
SWP_SYNCHRONOUS_IO devices", right in the critical part of
zswap_load().  Naive resolution of that conflict would have resulted in
basically reverting Johannes's fix.

That fix is cc:stable so we do want it to have a clean run in
linux-next before sending it upstream.  So I'll drop this patch
("zswap: replace RB tree with xarray") for now.  Please redo it against
latest mm-unstable and of course, be sure to preserve Johannes's fix,
thanks.