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

Peter Xu
2021-09-02T20:18:42Z

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