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, 18 Feb 2025 14:55:24 +1100
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.
Link: https://lkml.kernel.org/r/c22f699202db0acee2f7039eb026e68261ce42d6.1739850794.git-series.apopple@xxxxxxxxxx
Signed-off-by: Alistair Popple <apopple@xxxxxxxxxx>
Tested-by: Alison Schofield <alison.schofield@xxxxxxxxx>
Cc: Alexander Gordeev <agordeev@xxxxxxxxxxxxx>
Cc: Asahi Lina <lina@xxxxxxxxxxxxx>
Cc: Balbir Singh <balbirs@xxxxxxxxxx>
Cc: Bjorn Helgaas <bhelgaas@xxxxxxxxxx>
Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
Cc: Christian Borntraeger <borntraeger@xxxxxxxxxxxxx>
Cc: Christoph Hellwig <hch@xxxxxx>
Cc: Chunyan Zhang <zhang.lyra@xxxxxxxxx>
Cc: Dan Wiliams <dan.j.williams@xxxxxxxxx>
Cc: "Darrick J. Wong" <djwong@xxxxxxxxxx>
Cc: Dave Chinner <david@xxxxxxxxxxxxx>
Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Cc: Dave Jiang <dave.jiang@xxxxxxxxx>
Cc: David Hildenbrand <david@xxxxxxxxxx>
Cc: Gerald Schaefer <gerald.schaefer@xxxxxxxxxxxxx>
Cc: Heiko Carstens <hca@xxxxxxxxxxxxx>
Cc: Huacai Chen <chenhuacai@xxxxxxxxxx>
Cc: Ira Weiny <ira.weiny@xxxxxxxxx>
Cc: Jan Kara <jack@xxxxxxx>
Cc: Jason Gunthorpe <jgg@xxxxxxxxxx>
Cc: Jason Gunthorpe <jgg@xxxxxxxx>
Cc: John Hubbard <jhubbard@xxxxxxxxxx>
Cc: Logan Gunthorpe <logang@xxxxxxxxxxxx>
Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
Cc: Miaohe Lin <linmiaohe@xxxxxxxxxx>
Cc: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
Cc: Nicholas Piggin <npiggin@xxxxxxxxx>
Cc: Peter Xu <peterx@xxxxxxxxxx>
Cc: Sven Schnelle <svens@xxxxxxxxxxxxx>
Cc: Ted Ts'o <tytso@xxxxxxx>
Cc: Vasily Gorbik <gor@xxxxxxxxxxxxx>
Cc: Vishal Verma <vishal.l.verma@xxxxxxxxx>
Cc: Vivek Goyal <vgoyal@xxxxxxxxxx>
Cc: WANG Xuerui <kernel@xxxxxxxxxx>
Cc: Will Deacon <will@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
fs/dax.c | 55 +++++++++++++++++++++--------------
include/linux/page-flags.h | 6 ---
2 files changed, 33 insertions(+), 28 deletions(-)
--- a/fs/dax.c~fs-dax-remove-page_mapping_dax_shared-mapping-flag
+++ a/fs/dax.c
@@ -351,27 +351,40 @@ static unsigned long dax_end_pfn(void *e
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_sh
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 *en
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;
--- a/include/linux/page-flags.h~fs-dax-remove-page_mapping_dax_shared-mapping-flag
+++ a/include/linux/page-flags.h
@@ -710,12 +710,6 @@ 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
mm-rmap-add-support-for-pud-sized-mappings-to-rmap.patch
mm-huge_memory-add-vmf_insert_folio_pud.patch
mm-huge_memory-add-vmf_insert_folio_pmd.patch
mm-gup-dont-allow-foll_longterm-pinning-of-fs-dax-pages.patch
fs-dax-properly-refcount-fs-dax-pages.patch
device-dax-properly-refcount-device-dax-pages-when-mapping.patch
