[PATCH v6 03/13] mm/vmemmap: Improve vmemmap_can_optimize and allow architectures to override

[Aneesh Kumar K.V]

dax vmemmap optimization requires a minimum of 2 PAGE_SIZE area within
vmemmap such that tail page mapping can point to the second PAGE_SIZE area.
Enforce that in vmemmap_can_optimize() function.

Architectures like powerpc also want to enable vmemmap optimization
conditionally (only with radix MMU translation). Hence allow architecture
override.

Reviewed-by: Christophe Leroy <christophe.le...@csgroup.eu>
Signed-off-by: Aneesh Kumar K.V <aneesh.ku...@linux.ibm.com>
---
 include/linux/mm.h | 27 +++++++++++++++++++++++----
 mm/mm_init.c       |  2 +-
 2 files changed, 24 insertions(+), 5 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index a5d68baea231..fe6741539740 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -3649,13 +3649,32 @@ void vmemmap_free(unsigned long start, unsigned long 
end,
                struct vmem_altmap *altmap);
 #endif
 
+#define VMEMMAP_RESERVE_NR     2
 #ifdef CONFIG_ARCH_WANT_OPTIMIZE_VMEMMAP
-static inline bool vmemmap_can_optimize(struct vmem_altmap *altmap,
-                                          struct dev_pagemap *pgmap)
+static inline bool __vmemmap_can_optimize(struct vmem_altmap *altmap,
+                                         struct dev_pagemap *pgmap)
 {
-       return is_power_of_2(sizeof(struct page)) &&
-               pgmap && (pgmap_vmemmap_nr(pgmap) > 1) && !altmap;
+       unsigned long nr_pages;
+       unsigned long nr_vmemmap_pages;
+
+       if (!pgmap || !is_power_of_2(sizeof(struct page)))
+               return false;
+
+       nr_pages = pgmap_vmemmap_nr(pgmap);
+       nr_vmemmap_pages = ((nr_pages * sizeof(struct page)) >> PAGE_SHIFT);
+       /*
+        * For vmemmap optimization with DAX we need minimum 2 vmemmap
+        * pages. See layout diagram in Documentation/mm/vmemmap_dedup.rst
+        */
+       return !altmap && (nr_vmemmap_pages > VMEMMAP_RESERVE_NR);
 }
+/*
+ * If we don't have an architecture override, use the generic rule
+ */
+#ifndef vmemmap_can_optimize
+#define vmemmap_can_optimize __vmemmap_can_optimize
+#endif
+
 #else
 static inline bool vmemmap_can_optimize(struct vmem_altmap *altmap,
                                           struct dev_pagemap *pgmap)
diff --git a/mm/mm_init.c b/mm/mm_init.c
index acb0ac194672..641c56fd08a2 100644
--- a/mm/mm_init.c
+++ b/mm/mm_init.c
@@ -1020,7 +1020,7 @@ static inline unsigned long compound_nr_pages(struct 
vmem_altmap *altmap,
        if (!vmemmap_can_optimize(altmap, pgmap))
                return pgmap_vmemmap_nr(pgmap);
 
-       return 2 * (PAGE_SIZE / sizeof(struct page));
+       return VMEMMAP_RESERVE_NR * (PAGE_SIZE / sizeof(struct page));
 }
 
 static void __ref memmap_init_compound(struct page *head,
-- 
2.41.0