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

Peter Zijlstra
2022-01-20T16:09:17Z

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.
*/