2 files changed, 40 insertions, 0 deletions

diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index d16efa9066d9..590cff32415d 100644
--- a/include/linux/page-flags.h
+++ b/include/linux/page-flags.h
@@ -83,7 +83,12 @@ enum pageflags {
         PG_reserved,
         PG_private,             /* If pagecache, has fs-private data */
         PG_writeback,           /* Page is under writeback */
+#ifdef CONFIG_PAGEFLAGS_EXTENDED
+        PG_head,                /* A head page */
+        PG_tail,                /* A tail page */
+#else
         PG_compound,            /* A compound page */
+#endif
         PG_swapcache,           /* Swap page: swp_entry_t in private */
         PG_mappedtodisk,        /* Has blocks allocated on-disk */
         PG_reclaim,             /* To be reclaimed asap */
@@ -248,6 +253,28 @@ static inline void set_page_writeback(struct page *page)
         test_set_page_writeback(page);
 }
 
+#ifdef CONFIG_PAGEFLAGS_EXTENDED
+/*
+ * System with lots of page flags available. This allows separate
+ * flags for PageHead() and PageTail() checks of compound pages so that bit
+ * tests can be used in performance sensitive paths. PageCompound is
+ * generally not used in hot code paths.
+ */
+__PAGEFLAG(Head, head)
+__PAGEFLAG(Tail, tail)
+
+static inline int PageCompound(struct page *page)
+{
+        return page->flags & ((1L << PG_head) | (1L << PG_tail));
+
+}
+#else
+/*
+ * Reduce page flag use as much as possible by overlapping
+ * compound page flags with the flags used for page cache pages. Possible
+ * because PageCompound is always set for compound pages and not for
+ * pages on the LRU and/or pagecache.
+ */
 TESTPAGEFLAG(Compound, compound)
 __PAGEFLAG(Head, compound)
 
@@ -278,5 +305,6 @@ static inline void __ClearPageTail(struct page *page)
         page->flags &= ~PG_head_tail_mask;
 }
 
+#endif /* !PAGEFLAGS_EXTENDED */
 #endif /* !__GENERATING_BOUNDS_H */
 #endif  /* PAGE_FLAGS_H */
diff --git a/mm/Kconfig b/mm/Kconfig
index 0016ebd4dcba..3aa819d628c1 100644
--- a/mm/Kconfig
+++ b/mm/Kconfig
@@ -143,6 +143,18 @@ config MEMORY_HOTREMOVE
         depends on MEMORY_HOTPLUG && ARCH_ENABLE_MEMORY_HOTREMOVE
         depends on MIGRATION
 
+#
+# If we have space for more page flags then we can enable additional
+# optimizations and functionality.
+#
+# Regular Sparsemem takes page flag bits for the sectionid if it does not
+# use a virtual memmap. Disable extended page flags for 32 bit platforms
+# that require the use of a sectionid in the page flags.
+#
+config PAGEFLAGS_EXTENDED
+        def_bool y
+        depends on 64BIT || SPARSEMEM_VMEMMAP || !NUMA || !SPARSEMEM
+
 # Heavily threaded applications may benefit from splitting the mm-wide
 # page_table_lock, so that faults on different parts of the user address
 # space can be handled with less contention: split it at this NR_CPUS.