The patch titled
Subject: fs/dax: remove PAGE_MAPPING_DAX_SHARED mapping flag
has been added to the -mm mm-unstable branch. Its filename is
fs-dax-remove-page_mapping_dax_shared-mapping-flag.patch
This patch will shortly appear at
https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/fs-dax-remove-page_mapping_dax_shared-mapping-flag.patch
This patch will later appear in the mm-unstable branch at
git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
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------------------------------------------------------
From: Alistair Popple <apopple@xxxxxxxxxx>
Subject: fs/dax: remove PAGE_MAPPING_DAX_SHARED mapping flag
Date: Tue, 7 Jan 2025 14:42:24 +1100
PAGE_MAPPING_DAX_SHARED is the same as PAGE_MAPPING_ANON. This isn't
currently a problem because FS DAX pages are treated specially. 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.
Link: https://lkml.kernel.org/r/792742e7339dcc088fcbfb0daa9b5ab82534f557.1736221254.git-series.apopple@xxxxxxxxxx
Signed-off-by: Alistair Popple <apopple@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
fs/dax.c | 45 ++++++++++++++++++++++-------------
include/linux/page-flags.h | 6 ----
2 files changed, 29 insertions(+), 22 deletions(-)
--- a/fs/dax.c~fs-dax-remove-page_mapping_dax_shared-mapping-flag
+++ a/fs/dax.c
@@ -351,38 +351,41 @@ static unsigned long dax_end_pfn(void *e
for (pfn = dax_to_pfn(entry); \
pfn < dax_end_pfn(entry); pfn++)
+/*
+ * A DAX page 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 if shared between multiple files but has not yet actually been mapped
+ * into multiple address spaces.
+ */
static inline bool dax_page_is_shared(struct page *page)
{
- return page->mapping == PAGE_MAPPING_DAX_SHARED;
+ return !page->mapping && page->share;
}
/*
- * Set the page->mapping with PAGE_MAPPING_DAX_SHARED flag, increase the
- * refcount.
+ * Increase the page share refcount, warning if the page is not marked as shared.
*/
static inline void dax_page_share_get(struct page *page)
{
- if (page->mapping != PAGE_MAPPING_DAX_SHARED) {
- /*
- * Reset the index if the page was already mapped
- * regularly before.
- */
- if (page->mapping)
- page->share = 1;
- page->mapping = PAGE_MAPPING_DAX_SHARED;
- }
+ WARN_ON_ONCE(!page->share);
+ WARN_ON_ONCE(page->mapping);
page->share++;
}
static inline unsigned long dax_page_share_put(struct page *page)
{
+ WARN_ON_ONCE(!page->share);
return --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 page->mapping
- * PAGE_MAPPING_DAX_SHARED, and use page->share as refcount.
+ * When it is called in 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 the
+ * file-system to track which mappings contain which pages, ie. by implementing
+ * dax_holder_operations.
*/
static void dax_associate_entry(void *entry, struct address_space *mapping,
struct vm_area_struct *vma, unsigned long address, bool shared)
@@ -397,7 +400,17 @@ static void dax_associate_entry(void *en
for_each_mapped_pfn(entry, pfn) {
struct page *page = pfn_to_page(pfn);
- if (shared) {
+ if (shared && page->mapping && page->share) {
+ if (page->mapping) {
+ page->mapping = NULL;
+
+ /*
+ * Page has already been mapped into one address
+ * space so set the share count.
+ */
+ page->share = 1;
+ }
+
dax_page_share_get(page);
} else {
WARN_ON_ONCE(page->mapping);
--- a/include/linux/page-flags.h~fs-dax-remove-page_mapping_dax_shared-mapping-flag
+++ a/include/linux/page-flags.h
@@ -673,12 +673,6 @@ PAGEFLAG_FALSE(VmemmapSelfHosted, vmemma
#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;
_
Patches currently in -mm which might be from apopple@xxxxxxxxxx are
fuse-fix-dax-truncate-punch_hole-fault-path.patch
fs-dax-return-unmapped-busy-pages-from-dax_layout_busy_page_range.patch
fs-dax-dont-skip-locked-entries-when-scanning-entries.patch
fs-dax-refactor-wait-for-dax-idle-page.patch
fs-dax-create-a-common-implementation-to-break-dax-layouts.patch
fs-dax-always-remove-dax-page-cache-entries-when-breaking-layouts.patch
fs-dax-ensure-all-pages-are-idle-prior-to-filesystem-unmount.patch
fs-dax-remove-page_mapping_dax_shared-mapping-flag.patch
mm-gup-remove-redundant-check-for-pci-p2pdma-page.patch
mm-mm_init-move-p2pdma-page-refcount-initialisation-to-p2pdma.patch
mm-allow-compound-zone-device-pages.patch
mm-memory-enhance-insert_page_into_pte_locked-to-create-writable-mappings.patch
mm-memory-add-vmf_insert_page_mkwrite.patch
rmap-add-support-for-pud-sized-mappings-to-rmap.patch
huge_memory-add-vmf_insert_folio_pud.patch
huge_memory-add-vmf_insert_folio_pmd.patch
memremap-add-is_devdax_page-and-is_fsdax_page-helpers.patch
mm-gup-dont-allow-foll_longterm-pinning-of-fs-dax-pages.patch
proc-task_mmu-mark-devdax-and-fsdax-pages-as-always-unpinned.patch
mm-mlock-skip-zone_device-pmds-during-mlock.patch
fs-dax-properly-refcount-fs-dax-pages.patch
device-dax-properly-refcount-device-dax-pages-when-mapping.patch
mm-remove-pxx_devmap-callers.patch
mm-remove-devmap-related-functions-and-page-table-bits.patch
revert-riscv-mm-add-support-for-zone_device.patch
