mm/page_ext: do not allocate space for page_ext->flags if not needed

There is 8 byte page_ext->flags field allocated per page whenever
CONFIG_PAGE_EXTENSION is enabled.  However, not every user of page_ext
uses flags.  Therefore, check whether flags is needed at least by one user
and if so allocate space for it.

For example when page_table_check is enabled, on a machine with 128G
of memory before the fix:

[    2.244288] allocated 536870912 bytes of page_ext
after the fix:
[    2.160154] allocated 268435456 bytes of page_ext

Also, add a kernel-doc comment before page_ext_operations that describes
the fields, and remove check if need() is set, as that is now a required
field.

[pasha.tatashin@soleen.com: address comments from Mike Rapoport]
  Link: https://lkml.kernel.org/r/20230117202103.1412449-1-pasha.tatashin@soleen.com
Link: https://lkml.kernel.org/r/20230113154253.92480-1-pasha.tatashin@soleen.com
Signed-off-by: Pasha Tatashin <pasha.tatashin@soleen.com>
Acked-by: David Hildenbrand <david@redhat.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Acked-by: David Rientjes <rientjes@google.com>
Reviewed-by: Mike Rapoport (IBM) <rppt@kernel.org>
Cc: Charan Teja Kalla <quic_charante@quicinc.com>
Cc: Li Zhe <lizhe.67@bytedance.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 18 insertions, 0 deletions

diff --git a/include/linux/page_ext.h b/include/linux/page_ext.h
index 22be4582faae..67314f648aeb 100644
--- a/include/linux/page_ext.h
+++ b/include/linux/page_ext.h
@@ -7,15 +7,33 @@
 #include <linux/stackdepot.h>
 
 struct pglist_data;
+
+/**
+ * struct page_ext_operations - per page_ext client operations
+ * @offset: Offset to the client's data within page_ext. Offset is returned to
+ *          the client by page_ext_init.
+ * @size: The size of the client data within page_ext.
+ * @need: Function that returns true if client requires page_ext.
+ * @init: (optional) Called to initialize client once page_exts are allocated.
+ * @need_shared_flags: True when client is using shared page_ext->flags
+ *                     field.
+ *
+ * Each Page Extension client must define page_ext_operations in
+ * page_ext_ops array.
+ */
 struct page_ext_operations {
 	size_t offset;
 	size_t size;
 	bool (*need)(void);
 	void (*init)(void);
+	bool need_shared_flags;
 };
 
 #ifdef CONFIG_PAGE_EXTENSION
 
+/*
+ * The page_ext_flags users must set need_shared_flags to true.
+ */
 enum page_ext_flags {
 	PAGE_EXT_OWNER,
 	PAGE_EXT_OWNER_ALLOCATED,