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, 12 insertions, 2 deletions

diff --git a/mm/page_ext.c b/mm/page_ext.c
index 4ee522fd381c..e2c22ffdbb81 100644
--- a/mm/page_ext.c
+++ b/mm/page_ext.c
@@ -71,6 +71,7 @@ static bool need_page_idle(void)
 }
 static struct page_ext_operations page_idle_ops __initdata = {
 	.need = need_page_idle,
+	.need_shared_flags = true,
 };
 #endif
 
@@ -86,7 +87,7 @@ static struct page_ext_operations *page_ext_ops[] __initdata = {
 #endif
 };
 
-unsigned long page_ext_size = sizeof(struct page_ext);
+unsigned long page_ext_size;
 
 static unsigned long total_usage;
 static struct page_ext *lookup_page_ext(const struct page *page);
@@ -106,7 +107,16 @@ static bool __init invoke_need_callbacks(void)
 	bool need = false;
 
 	for (i = 0; i < entries; i++) {
-		if (page_ext_ops[i]->need && page_ext_ops[i]->need()) {
+		if (page_ext_ops[i]->need()) {
+			if (page_ext_ops[i]->need_shared_flags) {
+				page_ext_size = sizeof(struct page_ext);
+				break;
+			}
+		}
+	}
+
+	for (i = 0; i < entries; i++) {
+		if (page_ext_ops[i]->need()) {
 			page_ext_ops[i]->offset = page_ext_size;
 			page_ext_size += page_ext_ops[i]->size;
 			need = true;