mm: zswap: remove same_filled module params

These knobs offer more fine-grained control to userspace than needed and
directly expose/influence kernel implementation; remove them.

For disabling same_filled handling, there is no logical reason to refuse
storing same-filled pages more efficiently and opt for compression.
Scanning pages for patterns may be an argument, but the page contents will
be read into the CPU cache anyway during compression. Also, removing the
same_filled handling code does not move the needle significantly in terms
of performance anyway [1].

For disabling non_same_filled handling, it was added when the compressed
pages in zswap were not being properly charged to memcgs, as workloads
could escape the accounting with compression [2]. This is no longer the
case after commit f4840ccfca25 ("zswap: memcg accounting"), and using
zswap without compression does not make much sense.

[1]https://lore.kernel.org/lkml/CAJD7tkaySFP2hBQw4pnZHJJwe3bMdjJ1t9VC2VJd=[e-mail]/
[2]https://lore.kernel.org/lkml/[e-mail]/

[[e-mail]: remove same_filled_pages from docs]
Link: https://lkml.kernel.org/r/[e-mail]
Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: Yosry Ahmed <[e-mail]>
Acked-by: Johannes Weiner <[e-mail]>
Reviewed-by: Nhat Pham <[e-mail]>
Reviewed-by: Chengming Zhou <[e-mail]>
Cc: "Maciej S. Szmigiero" <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

Documentation/admin-guide/mm/zswap.rst

···
 compressed length of the page is set to zero and the pattern or same-filled
 value is stored.
 
-Same-value filled pages identification feature is enabled by default and can be
-disabled at boot time by setting the ``same_filled_pages_enabled`` attribute
-to 0, e.g. ``zswap.same_filled_pages_enabled=0``. It can also be enabled and
-disabled at runtime using the sysfs ``same_filled_pages_enabled``
-attribute, e.g.::
-
-	echo 1 > /sys/module/zswap/parameters/same_filled_pages_enabled
-
-When zswap same-filled page identification is disabled at runtime, it will stop
-checking for the same-value filled pages during store operation.
-In other words, every page will be then considered non-same-value filled.
-However, the existing pages which are marked as same-value filled pages remain
-stored unchanged in zswap until they are either loaded or invalidated.
-
-In some circumstances it might be advantageous to make use of just the zswap
-ability to efficiently store same-filled pages without enabling the whole
-compressed page storage.
-In this case the handling of non-same-value pages by zswap (enabled by default)
-can be disabled by setting the ``non_same_filled_pages_enabled`` attribute
-to 0, e.g. ``zswap.non_same_filled_pages_enabled=0``.
-It can also be enabled and disabled at runtime using the sysfs
-``non_same_filled_pages_enabled`` attribute, e.g.::
-
-	echo 1 > /sys/module/zswap/parameters/non_same_filled_pages_enabled
-
-Disabling both ``zswap.same_filled_pages_enabled`` and
-``zswap.non_same_filled_pages_enabled`` effectively disables accepting any new
-pages by zswap.
-
 To prevent zswap from shrinking pool when zswap is full and there's a high
 pressure on swap (this will result in flipping pages in and out zswap pool
 without any real benefit but with a performance drop for the system), a

Documentation/driver-api/crypto/iaa/iaa-crypto.rst

···
   # echo deflate-iaa > /sys/module/zswap/parameters/compressor
   # echo zsmalloc > /sys/module/zswap/parameters/zpool
   # echo 1 > /sys/module/zswap/parameters/enabled
-  # echo 0 > /sys/module/zswap/parameters/same_filled_pages_enabled
   # echo 100 > /proc/sys/vm/swappiness
   # echo never > /sys/kernel/mm/transparent_hugepage/enabled
   # echo 1 > /proc/sys/vm/overcommit_memory
···
   echo deflate-iaa > /sys/module/zswap/parameters/compressor
   echo zsmalloc > /sys/module/zswap/parameters/zpool
   echo 1 > /sys/module/zswap/parameters/enabled
-  echo 0 > /sys/module/zswap/parameters/same_filled_pages_enabled
 
   echo 100 > /proc/sys/vm/swappiness
   echo never > /sys/kernel/mm/transparent_hugepage/enabled

mm/zswap.c

···
 module_param_named(accept_threshold_percent, zswap_accept_thr_percent,
 		   uint, 0644);
 
-/*
- * Enable/disable handling same-value filled pages (enabled by default).
- * If disabled every page is considered non-same-value filled.
- */
-static bool zswap_same_filled_pages_enabled = true;
-module_param_named(same_filled_pages_enabled, zswap_same_filled_pages_enabled,
-		   bool, 0644);
-
-/* Enable/disable handling non-same-value filled pages (enabled by default) */
-static bool zswap_non_same_filled_pages_enabled = true;
-module_param_named(non_same_filled_pages_enabled, zswap_non_same_filled_pages_enabled,
-		   bool, 0644);
-
 /* Number of zpools in zswap_pool (empirically determined for scalability) */
 #define ZSWAP_NR_ZPOOLS 32
 
···
 	unsigned int pos, last_pos = PAGE_SIZE / sizeof(*page) - 1;
 	bool ret = false;
 
-	if (!zswap_same_filled_pages_enabled)
-		return false;
-
 	page = kmap_local_folio(folio, 0);
 	val = page[0];
 
···
 		atomic_inc(&zswap_same_filled_pages);
 		goto store_entry;
 	}
-
-	if (!zswap_non_same_filled_pages_enabled)
-		goto freepage;
 
 	/* if entry is successfully added, it keeps the reference */
 	entry->pool = zswap_pool_current_get();