mm: thp: optimize compound_trans_huge

Currently we don't clobber page_tail->first_page during split_huge_page,
so compound_trans_head can be set to compound_head without adverse
effects, and this mostly optimizes away a smp_rmb.

It looks worthwhile to keep around the implementation that doesn't relay
on page_tail->first_page not to be clobbered, because it would be
necessary if we'll decide to enforce page->private to zero at all times
whenever PG_private is not set, also for anonymous pages. For anonymous
pages enforcing such an invariant doesn't matter as anonymous pages
don't use page->private so we can get away with this microoptimization.

Signed-off-by: Andrea Arcangeli <[e-mail]>
Cc: Khalid Aziz <[e-mail]>
Cc: Pravin Shelar <[e-mail]>
Cc: Greg Kroah-Hartman <[e-mail]>
Cc: Ben Hutchings <[e-mail]>
Cc: Christoph Lameter <[e-mail]>
Cc: Johannes Weiner <[e-mail]>
Cc: Mel Gorman <[e-mail]>
Cc: Rik van Riel <[e-mail]>
Cc: Andi Kleen <[e-mail]>
Cc: Minchan Kim <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

include/linux/huge_mm.h

···
 		return HPAGE_PMD_NR;
 	return 1;
 }
+/*
+ * compound_trans_head() should be used instead of compound_head(),
+ * whenever the "page" passed as parameter could be the tail of a
+ * transparent hugepage that could be undergoing a
+ * __split_huge_page_refcount(). The page structure layout often
+ * changes across releases and it makes extensive use of unions. So if
+ * the page structure layout will change in a way that
+ * page->first_page gets clobbered by __split_huge_page_refcount, the
+ * implementation making use of smp_rmb() will be required.
+ *
+ * Currently we define compound_trans_head as compound_head, because
+ * page->private is in the same union with page->first_page, and
+ * page->private isn't clobbered. However this also means we're
+ * currently leaving dirt into the page->private field of anonymous
+ * pages resulting from a THP split, instead of setting page->private
+ * to zero like for every other page that has PG_private not set. But
+ * anonymous pages don't use page->private so this is not a problem.
+ */
+#if 0
+/* This will be needed if page->private will be clobbered in split_huge_page */
 static inline struct page *compound_trans_head(struct page *page)
 {
 	if (PageTail(page)) {
···
 	}
 	return page;
 }
+#else
+#define compound_trans_head(page) compound_head(page)
+#endif
 
 extern int do_huge_pmd_numa_page(struct mm_struct *mm, struct vm_area_struct *vma,
 				unsigned long addr, pmd_t pmd, pmd_t *pmdp);