[PATCH v2 5/5] mm: Add ZAP_FLAG_SKIP_SWAP and zap_flags

[Peter Xu <peterx@redhat.com>]

Firstly, the comment in zap_pte_range() is misleading because it checks against
details rather than check_mappings, so it's against what the code did.

Meanwhile, it's confusing too on not explaining why passing in the details
pointer would mean to skip all swap entries.  New user of zap_details could
very possibly miss this fact if they don't read deep until zap_pte_range()
because there's no comment at zap_details talking about it at all, so swap
entries could be errornously skipped without being noticed.

Actually very recently we introduced unmap_mapping_page() in 22061a1ffabd, I
think that should also look into swap entries.  Add a comment there.  IOW, this
patch will be a functional change to unmap_mapping_page() but hopefully in the
right way to do it.

This partly reverts 3e8715fdc03e ("mm: drop zap_details::check_swap_entries"),
but introduce ZAP_FLAG_SKIP_SWAP flag, which means the opposite of previous
"details" parameter: the caller should explicitly set this to skip swap
entries, otherwise swap entries will always be considered (which should still
be the major case here).

Cc: Kirill A. Shutemov <kirill@shutemov.name>
Cc: Hugh Dickins <hughd@google.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
---
 include/linux/mm.h | 16 ++++++++++++++++
 mm/memory.c        |  6 +++---
 2 files changed, 19 insertions(+), 3 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 81e402a5fbc9..a7bcdb2ec956 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1716,12 +1716,18 @@ static inline bool can_do_mlock(void) { return false; }
 extern int user_shm_lock(size_t, struct ucounts *);
 extern void user_shm_unlock(size_t, struct ucounts *);
 
+typedef unsigned int __bitwise zap_flags_t;
+
+/* Whether to skip zapping swap entries */
+#define  ZAP_FLAG_SKIP_SWAP  ((__force zap_flags_t) BIT(0))
+
 /*
  * Parameter block passed down to zap_pte_range in exceptional cases.
  */
 struct zap_details {
 	struct address_space *zap_mapping;	/* Check page->mapping if set */
 	struct page *single_page;		/* Locked page to be unmapped */
+	zap_flags_t zap_flags;			/* Extra flags for zapping */
 };
 
 /*
@@ -1737,6 +1743,16 @@ zap_skip_check_mapping(struct zap_details *details, struct page *page)
 	return details->zap_mapping != page_rmapping(page);
 }
 
+/* Return true if skip swap entries, false otherwise */
+static inline bool
+zap_skip_swap(struct zap_details *details)
+{
+	if (!details)
+		return false;
+
+	return details->zap_flags & ZAP_FLAG_SKIP_SWAP;
+}
+
 struct page *vm_normal_page(struct vm_area_struct *vma, unsigned long addr,
 			     pte_t pte);
 struct page *vm_normal_page_pmd(struct vm_area_struct *vma, unsigned long addr,
diff --git a/mm/memory.c b/mm/memory.c
index e5ee8399d270..4cb269ca8249 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -1379,8 +1379,7 @@ static unsigned long zap_pte_range(struct mmu_gather *tlb,
 			continue;
 		}
 
-		/* If details->check_mapping, we leave swap entries. */
-		if (unlikely(details))
+		if (unlikely(zap_skip_swap(details)))
 			continue;
 
 		if (!non_swap_entry(entry))
@@ -3351,6 +3350,7 @@ void unmap_mapping_page(struct page *page)
 	first_index = page->index;
 	last_index = page->index + thp_nr_pages(page) - 1;
 
+	/* Keep ZAP_FLAG_SKIP_SWAP cleared because we're truncating */
 	details.zap_mapping = mapping;
 	details.single_page = page;
 
@@ -3377,7 +3377,7 @@ void unmap_mapping_pages(struct address_space *mapping, pgoff_t start,
 		pgoff_t nr, bool even_cows)
 {
 	pgoff_t	first_index = start, last_index = start + nr - 1;
-	struct zap_details details = { };
+	struct zap_details details = { .zap_flags = ZAP_FLAG_SKIP_SWAP };
 
 	details.zap_mapping = even_cows ? NULL : mapping;
 	if (last_index < first_index)
-- 
2.31.1