mm: add support for discard of unused ptes

In a virtualized environment and given an appropriate interface the guest can mark pages as unused while they are free (for the s390 implementation see git commit 45e576b1c3d00206 "guest page hinting light"). For the host the unused state is a property of the pte. This patch adds the primitive 'pte_unused' and code to the host swap out handler so that pages marked as unused by all mappers are not swapped out but discarded instead, thus saving one IO for swap out and potentially another one for swap in. [ Martin Schwidefsky: patch reordering and simplification ] Signed-off-by: Konstantin Weitz <konstantin.weitz@gmail.com> Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

mm: add support for discard of unused ptes
In a virtualized environment and given an appropriate interface the guest can mark pages as unused while they are free (for the s390 implementation see git commit 45e576b1c3d00206 "guest page hinting light"). For the host the unused state is a property of the pte. This patch adds the primitive 'pte_unused' and code to the host swap out handler so that pages marked as unused by all mappers are not swapped out but discarded instead, thus saving one IO for swap out and potentially another one for swap in. [ Martin Schwidefsky: patch reordering and simplification ] Signed-off-by: Konstantin Weitz <konstantin.weitz@gmail.com> Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
Konstantin Weitz · Martin Schwidefsky
1 parent 53e857f308
Showing 2 changed files with 23 additions and 0 deletions Side-by-side Diff
include/asm-generic/pgtable.h
mm/rmap.c
@@ -193,6 +193,19 @@
 }
 #endif
  
+#ifndef __HAVE_ARCH_PTE_UNUSED
+/*
+ * Some architectures provide facilities to virtualization guests
+ * so that they can flag allocated pages as unused. This allows the
+ * host to transparently reclaim unused pages. This function returns
+ * whether the pte's page is unused.
+ */
+static inline int pte_unused(pte_t pte)
+{
+	return 0;
+}
+#endif
+
 #ifndef __HAVE_ARCH_PMD_SAME
 #ifdef CONFIG_TRANSPARENT_HUGEPAGE
 static inline int pmd_same(pmd_t pmd_a, pmd_t pmd_b)
@@ -1165,6 +1165,16 @@
 		}
 		set_pte_at(mm, address, pte,
 			   swp_entry_to_pte(make_hwpoison_entry(page)));
+	} else if (pte_unused(pteval)) {
+		/*
+		 * The guest indicated that the page content is of no
+		 * interest anymore. Simply discard the pte, vmscan
+		 * will take care of the rest.
+		 */
+		if (PageAnon(page))
+			dec_mm_counter(mm, MM_ANONPAGES);
+		else
+			dec_mm_counter(mm, MM_FILEPAGES);
 	} else if (PageAnon(page)) {
 		swp_entry_t entry = { .val = page_private(page) };
 		pte_t swp_pte;
...	...	@@ -193,6 +193,19 @@
193	193	}
194	194	#endif
195	195
	196	+#ifndef __HAVE_ARCH_PTE_UNUSED
	197	+/*
	198	+ * Some architectures provide facilities to virtualization guests
	199	+ * so that they can flag allocated pages as unused. This allows the
	200	+ * host to transparently reclaim unused pages. This function returns
	201	+ * whether the pte's page is unused.
	202	+ */
	203	+static inline int pte_unused(pte_t pte)
	204	+{
	205	+ return 0;
	206	+}
	207	+#endif
	208	+
196	209	#ifndef __HAVE_ARCH_PMD_SAME
197	210	#ifdef CONFIG_TRANSPARENT_HUGEPAGE
198	211	static inline int pmd_same(pmd_t pmd_a, pmd_t pmd_b)
...	...	@@ -1165,6 +1165,16 @@
1165	1165	}
1166	1166	set_pte_at(mm, address, pte,
1167	1167	swp_entry_to_pte(make_hwpoison_entry(page)));
	1168	+ } else if (pte_unused(pteval)) {
	1169	+ /*
	1170	+ * The guest indicated that the page content is of no
	1171	+ * interest anymore. Simply discard the pte, vmscan
	1172	+ * will take care of the rest.
	1173	+ */
	1174	+ if (PageAnon(page))
	1175	+ dec_mm_counter(mm, MM_ANONPAGES);
	1176	+ else
	1177	+ dec_mm_counter(mm, MM_FILEPAGES);
1168	1178	} else if (PageAnon(page)) {
1169	1179	swp_entry_t entry = { .val = page_private(page) };
1170	1180	pte_t swp_pte;