[PATCH v8 08/20] fs/dax: Remove PAGE_MAPPING_DAX_SHARED mapping flag

Alistair Popple
2025-02-18T03:56:37Z

The page ->mapping pointer can have magic values like
PAGE_MAPPING_DAX_SHARED and PAGE_MAPPING_ANON for page owner specific
usage. Currently PAGE_MAPPING_DAX_SHARED and PAGE_MAPPING_ANON alias to the
same value. This isn't a problem because FS DAX pages are never seen by the
anonymous mapping code and vice versa.

However a future change will make FS DAX pages more like normal pages, so
folio_test_anon() must not return true for a FS DAX page.

We could explicitly test for a FS DAX page in folio_test_anon(),
etc. however the PAGE_MAPPING_DAX_SHARED flag isn't actually
needed. Instead we can use the page->mapping field to implicitly track the
first mapping of a page. If page->mapping is non-NULL it implies the page
is associated with a single mapping at page->index. If the page is
associated with a second mapping clear page->mapping and set page->share to
1.

This is possible because a shared mapping implies the file-system
implements dax_holder_operations which makes the ->mapping and ->index,
which is a union with ->share, unused.

The page is considered shared when page->mapping == NULL and
page->share > 0 or page->mapping != NULL, implying it is present in at
least one address space. This also makes it easier for a future change to
detect when a page is first mapped into an address space which requires
special handling.

Signed-off-by: Alistair Popple <apopple@nvidia.com>

---

Changes for v8:

- Rebased on mm-unstable which includes Matthew Wilcox's "dax: use
folios more widely within DAX"

Changes for v7:

- Fix for checking when creating a shared mapping in dax_associate_entry.
- Remove dax_page_share_get().
- Add dax_page_make_shared().
---
fs/dax.c | 55 +++++++++++++++++++++++----------------
include/linux/page-flags.h | 6 +----
2 files changed, 33 insertions(+), 28 deletions(-)

diff --git a/fs/dax.c b/fs/dax.c
index bc538ba..6674540 100644
--- a/fs/dax.c
+++ b/fs/dax.c
@@ -351,27 +351,40 @@ static unsigned long dax_end_pfn(void *entry)
for (pfn = dax_to_pfn(entry); \
pfn < dax_end_pfn(entry); pfn++)

+/*
+ * A DAX folio is considered shared if it has no mapping set and ->share (which
+ * shares the ->index field) is non-zero. Note this may return false even if the
+ * page is shared between multiple files but has not yet actually been mapped
+ * into multiple address spaces.
+ */
static inline bool dax_folio_is_shared(struct folio *folio)
{
- return folio->mapping == PAGE_MAPPING_DAX_SHARED;
+ return !folio->mapping && folio->page.share;
}

/*
- * Set the folio->mapping with PAGE_MAPPING_DAX_SHARED flag, increase the
- * refcount.
+ * When it is called by dax_insert_entry(), the shared flag will indicate
+ * whether this entry is shared by multiple files. If the page has not
+ * previously been associated with any mappings the ->mapping and ->index
+ * fields will be set. If it has already been associated with a mapping
+ * the mapping will be cleared and the share count set. It's then up to
+ * reverse map users like memory_failure() to call back into the filesystem to
+ * recover ->mapping and ->index information. For example by implementing
+ * dax_holder_operations.
*/
-static inline void dax_folio_share_get(struct folio *folio)
+static void dax_folio_make_shared(struct folio *folio)
{
- if (folio->mapping != PAGE_MAPPING_DAX_SHARED) {
- /*
- * Reset the index if the page was already mapped
- * regularly before.
- */
- if (folio->mapping)
- folio->page.share = 1;
- folio->mapping = PAGE_MAPPING_DAX_SHARED;
- }
- folio->page.share++;
+ /*
+ * folio is not currently shared so mark it as shared by clearing
+ * folio->mapping.
+ */
+ folio->mapping = NULL;
+
+ /*
+ * folio has previously been mapped into one address space so set the
+ * share count.
+ */
+ folio->page.share = 1;
}

static inline unsigned long dax_folio_share_put(struct folio *folio)
@@ -379,12 +392,6 @@ static inline unsigned long dax_folio_share_put(struct folio *folio)
return --folio->page.share;
}

-/*
- * When it is called in dax_insert_entry(), the shared flag will indicate
- * that whether this entry is shared by multiple files. If so, set
- * the folio->mapping PAGE_MAPPING_DAX_SHARED, and use page->share
- * as refcount.
- */
static void dax_associate_entry(void *entry, struct address_space *mapping,
struct vm_area_struct *vma, unsigned long address, bool shared)
{
@@ -398,8 +405,12 @@ static void dax_associate_entry(void *entry, struct address_space *mapping,
for_each_mapped_pfn(entry, pfn) {
struct folio *folio = pfn_folio(pfn);

- if (shared) {
- dax_folio_share_get(folio);
+ if (shared && (folio->mapping || folio->page.share)) {
+ if (folio->mapping)
+ dax_folio_make_shared(folio);
+
+ WARN_ON_ONCE(!folio->page.share);
+ folio->page.share++;
} else {
WARN_ON_ONCE(folio->mapping);
folio->mapping = mapping;
diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index 3f6a64f..30fe3eb 100644
--- a/include/linux/page-flags.h
+++ b/include/linux/page-flags.h
@@ -710,12 +710,6 @@ PAGEFLAG_FALSE(VmemmapSelfHosted, vmemmap_self_hosted)
#define PAGE_MAPPING_KSM (PAGE_MAPPING_ANON | PAGE_MAPPING_MOVABLE)
#define PAGE_MAPPING_FLAGS (PAGE_MAPPING_ANON | PAGE_MAPPING_MOVABLE)

-/*
- * Different with flags above, this flag is used only for fsdax mode. It
- * indicates that this page->mapping is now under reflink case.
- */
-#define PAGE_MAPPING_DAX_SHARED ((void *)0x1)
-
static __always_inline bool folio_mapping_flags(const struct folio *folio)
{
return ((unsigned long)folio->mapping & PAGE_MAPPING_FLAGS) != 0;
--
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