mm: z3fold: deprecate CONFIG_Z3FOLD

The z3fold compressed pages allocator is rarely used, most users use
zsmalloc.  The only disadvantage of zsmalloc in comparison is the
dependency on MMU, and zbud is a more common option for !MMU as it was the
default zswap allocator for a long time.

Historically, zsmalloc had worse latency than zbud and z3fold but offered
better memory savings.  This is no longer the case as shown by a simple
recent analysis [1].  That analysis showed that z3fold does not have any
advantage over zsmalloc or zbud considering both performance and memory
usage.  In a kernel build test on tmpfs in a limited cgroup, z3fold took
3% more time and used 1.8% more memory.  The latency of zswap_load() was
7% higher, and that of zswap_store() was 10% higher.  Zsmalloc is better
in all metrics.

Moreover, z3fold apparently has latent bugs, which was made noticeable by
a recent soft lockup bug report with z3fold [2].  Switching to zsmalloc
not only fixed the problem, but also reduced the swap usage from 6~8G to
1~2G.  Other users have also reported being bitten by mistakenly enabling
z3fold.

Other than hurting users, z3fold is repeatedly causing wasted engineering
effort.  Apart from investigating the above bug, it came up in multiple
development discussions (e.g.  [3]) as something we need to handle, when
there aren't any legit users (at least not intentionally).

The natural course of action is to deprecate z3fold, and remove in a few
cycles if no objections are raised from active users.  Next on the list
should be zbud, as it offers marginal latency gains at the cost of huge
memory waste when compared to zsmalloc.  That one will need to wait until
zsmalloc does not depend on MMU.

Rename the user-visible config option from CONFIG_Z3FOLD to
CONFIG_Z3FOLD_DEPRECATED so that users with CONFIG_Z3FOLD=y get a new
prompt with explanation during make oldconfig.  Also, remove
CONFIG_Z3FOLD=y from defconfigs.

[1]https://lore.kernel.org/lkml/CAJD7tkbRF6od-2x_L8-A1QL3=2Ww13sCj4S3i4bNndqF+3+_Vg@mail.gmail.com/
[2]https://lore.kernel.org/lkml/[email protected]/
[3]https://lore.kernel.org/lkml/CAJD7tkbnmeVugfunffSovJf9FAgy9rhBVt_tx=nxUveLUfqVsA@mail.gmail.com/

[[email protected]: deprecate ZSWAP_ZPOOL_DEFAULT_Z3FOLD as well]
  Link: https://lkml.kernel.org/r/[email protected]
Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Yosry Ahmed <[email protected]>
Signed-off-by: Arnd Bergmann <[email protected]>
Acked-by: Chris Down <[email protected]>
Acked-by: Nhat Pham <[email protected]>
Acked-by: Johannes Weiner <[email protected]>
Acked-by: Vitaly Wool <[email protected]>
Acked-by: Christoph Hellwig <[email protected]>
Cc: Aneesh Kumar K.V <[email protected]>
Cc: Christophe Leroy <[email protected]>
Cc: Huacai Chen <[email protected]>
Cc: Miaohe Lin <[email protected]>
Cc: Michael Ellerman <[email protected]>
Cc: Naveen N. Rao <[email protected]>
Cc: Nicholas Piggin <[email protected]>
Cc: Sergey Senozhatsky <[email protected]>
Cc: WANG Xuerui <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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