mm/mmu_gather: define ENCODED_PAGE_FLAG_DELAY_RMAP

Nowadays, encoded pages are only used in mmu_gather handling.  Let's
update the documentation, and define ENCODED_PAGE_BIT_DELAY_RMAP.  While
at it, rename ENCODE_PAGE_BITS to ENCODED_PAGE_BITS.

If encoded page pointers would ever be used in other context again, we'd
likely want to change the defines to reflect their context (e.g.,
ENCODED_PAGE_FLAG_MMU_GATHER_DELAY_RMAP).  For now, let's keep it simple.

This is a preparation for using the remaining spare bit to indicate that
the next item in an array of encoded pages is a "nr_pages" argument and
not an encoded page.

Link: https://lkml.kernel.org/r/20240214204435.167852-7-david@redhat.com
Signed-off-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Ryan Roberts <ryan.roberts@arm.com>
Cc: Alexander Gordeev <agordeev@linux.ibm.com>
Cc: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Christian Borntraeger <borntraeger@linux.ibm.com>
Cc: Christophe Leroy <christophe.leroy@csgroup.eu>
Cc: Heiko Carstens <hca@linux.ibm.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Michal Hocko <mhocko@suse.com>
Cc: "Naveen N. Rao" <naveen.n.rao@linux.ibm.com>
Cc: Nicholas Piggin <npiggin@gmail.com>
Cc: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Sven Schnelle <svens@linux.ibm.com>
Cc: Vasily Gorbik <gor@linux.ibm.com>
Cc: Will Deacon <will@kernel.org>
Cc: Yin Fengwei <fengwei.yin@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 11 insertions, 6 deletions

diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 8b611e13153e..1b89eec0d6df 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -210,8 +210,8 @@ struct page {
  *
  * An 'encoded_page' pointer is a pointer to a regular 'struct page', but
  * with the low bits of the pointer indicating extra context-dependent
- * information. Not super-common, but happens in mmu_gather and mlock
- * handling, and this acts as a type system check on that use.
+ * information. Only used in mmu_gather handling, and this acts as a type
+ * system check on that use.
  *
  * We only really have two guaranteed bits in general, although you could
  * play with 'struct page' alignment (see CONFIG_HAVE_ALIGNED_STRUCT_PAGE)
@@ -220,21 +220,26 @@ struct page {
  * Use the supplied helper functions to endcode/decode the pointer and bits.
  */
 struct encoded_page;
-#define ENCODE_PAGE_BITS 3ul
+
+#define ENCODED_PAGE_BITS			3ul
+
+/* Perform rmap removal after we have flushed the TLB. */
+#define ENCODED_PAGE_BIT_DELAY_RMAP		1ul
+
 static __always_inline struct encoded_page *encode_page(struct page *page, unsigned long flags)
 {
-	BUILD_BUG_ON(flags > ENCODE_PAGE_BITS);
+	BUILD_BUG_ON(flags > ENCODED_PAGE_BITS);
 	return (struct encoded_page *)(flags | (unsigned long)page);
 }
 
 static inline unsigned long encoded_page_flags(struct encoded_page *page)
 {
-	return ENCODE_PAGE_BITS & (unsigned long)page;
+	return ENCODED_PAGE_BITS & (unsigned long)page;
 }
 
 static inline struct page *encoded_page_ptr(struct encoded_page *page)
 {
-	return (struct page *)(~ENCODE_PAGE_BITS & (unsigned long)page);
+	return (struct page *)(~ENCODED_PAGE_BITS & (unsigned long)page);
 }
 
 /*