[RFC][PATCH v2 1/5] mm: Avoid unmapping pinned pages

[Peter Zijlstra <peterz@infradead.org>]

Add a guarantee for Anon pages that pin_user_page*() ensures the
user-mapping of these pages stay preserved. In order to ensure this
all rmap users have been audited:

 vmscan:	already fails eviction due to page_maybe_dma_pinned()

 migrate:	migration will fail on pinned pages due to
		expected_page_refs() not matching, however that is
		*after* try_to_migrate() has already destroyed the
		user mapping of these pages. Add an early exit for
		this case.

 numa-balance:	as per the above, pinned pages cannot be migrated,
		however numa balancing scanning will happily PROT_NONE
		them to get usage information on these pages. Avoid
		this for pinned pages.

None of the other rmap users (damon,page-idle,mlock,..) unmap the
page, they mostly just muck about with reference,dirty flags etc.

This same guarantee cannot be provided for Shared (file) pages due to
dirty page tracking.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
 mm/migrate.c  |   10 +++++++++-
 mm/mprotect.c |    6 ++++++
 2 files changed, 15 insertions(+), 1 deletion(-)

--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -1472,7 +1472,15 @@ int migrate_pages(struct list_head *from
 			nr_subpages = thp_nr_pages(page);
 			cond_resched();
 
-			if (PageHuge(page))
+			/*
+			 * If the page has a pin then expected_page_refs() will
+			 * not match and the whole migration will fail later
+			 * anyway, fail early and preserve the mappings.
+			 */
+			if (page_maybe_dma_pinned(page))
+				rc = -EAGAIN;
+
+			else if (PageHuge(page))
 				rc = unmap_and_move_huge_page(get_new_page,
 						put_new_page, private, page,
 						pass > 2, mode, reason,
--- a/mm/mprotect.c
+++ b/mm/mprotect.c
@@ -106,6 +106,12 @@ static unsigned long change_pte_range(st
 					continue;
 
 				/*
+				 * Can't migrate pinned pages, avoid touching them.
+				 */
+				if (page_maybe_dma_pinned(page))
+					continue;
+
+				/*
 				 * Don't mess with PTEs if page is already on the node
 				 * a single-threaded process is running on.
 				 */