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720355d977c111da236da7cd52917f60ec1c9c53
linux/debian/patches/patchset-pf/fixes/0005-mm-add-folio_expected_ref_count-for-reference-count-.patch
Konstantin Demin 720355d977 release 6.15.10
2025-08-15 15:46:56 +03:00

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From 8135974e9e512fdf6d15f59947f95e44f2834c37 Mon Sep 17 00:00:00 2001
From: Shivank Garg <shivankg@amd.com>
Date: Wed, 30 Apr 2025 10:01:51 +0000
Subject: mm: add folio_expected_ref_count() for reference count calculation
Patch series " JFS: Implement migrate_folio for jfs_metapage_aops" v5.
This patchset addresses a warning that occurs during memory compaction due
to JFS's missing migrate_folio operation. The warning was introduced by
commit 7ee3647243e5 ("migrate: Remove call to ->writepage") which added
explicit warnings when filesystem don't implement migrate_folio.
The syzbot reported following [1]:
jfs_metapage_aops does not implement migrate_folio
WARNING: CPU: 1 PID: 5861 at mm/migrate.c:955 fallback_migrate_folio mm/migrate.c:953 [inline]
WARNING: CPU: 1 PID: 5861 at mm/migrate.c:955 move_to_new_folio+0x70e/0x840 mm/migrate.c:1007
Modules linked in:
CPU: 1 UID: 0 PID: 5861 Comm: syz-executor280 Not tainted 6.15.0-rc1-next-20250411-syzkaller #0 PREEMPT(full)
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 02/12/2025
RIP: 0010:fallback_migrate_folio mm/migrate.c:953 [inline]
RIP: 0010:move_to_new_folio+0x70e/0x840 mm/migrate.c:1007
To fix this issue, this series implement metapage_migrate_folio() for JFS
which handles both single and multiple metapages per page configurations.
While most filesystems leverage existing migration implementations like
filemap_migrate_folio(), buffer_migrate_folio_norefs() or
buffer_migrate_folio() (which internally used folio_expected_refs()),
JFS's metapage architecture requires special handling of its private data
during migration. To support this, this series introduce the
folio_expected_ref_count(), which calculates external references to a
folio from page/swap cache, private data, and page table mappings.
This standardized implementation replaces the previous ad-hoc
folio_expected_refs() function and enables JFS to accurately determine
whether a folio has unexpected references before attempting migration.
Implement folio_expected_ref_count() to calculate expected folio reference
counts from:
- Page/swap cache (1 per page)
- Private data (1)
- Page table mappings (1 per map)
While originally needed for page migration operations, this improved
implementation standardizes reference counting by consolidating all
refcount contributors into a single, reusable function that can benefit
any subsystem needing to detect unexpected references to folios.
The folio_expected_ref_count() returns the sum of these external
references without including any reference the caller itself might hold.
Callers comparing against the actual folio_ref_count() must account for
their own references separately.
Link: https://syzkaller.appspot.com/bug?extid=8bb6fd945af4e0ad9299 [1]
Link: https://lkml.kernel.org/r/20250430100150.279751-1-shivankg@amd.com
Link: https://lkml.kernel.org/r/20250430100150.279751-2-shivankg@amd.com
Signed-off-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Shivank Garg <shivankg@amd.com>
Suggested-by: Matthew Wilcox <willy@infradead.org>
Co-developed-by: David Hildenbrand <david@redhat.com>
Cc: Alistair Popple <apopple@nvidia.com>
Cc: Dave Kleikamp <shaggy@kernel.org>
Cc: Donet Tom <donettom@linux.ibm.com>
Cc: Jane Chu <jane.chu@oracle.com>
Cc: Kefeng Wang <wangkefeng.wang@huawei.com>
Cc: Zi Yan <ziy@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---
include/linux/mm.h | 55 ++++++++++++++++++++++++++++++++++++++++++++++
mm/migrate.c | 22 ++++---------------
2 files changed, 59 insertions(+), 18 deletions(-)
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -2337,6 +2337,61 @@ static inline bool folio_maybe_mapped_sh
return folio_test_large_maybe_mapped_shared(folio);
}
+/**
+ * folio_expected_ref_count - calculate the expected folio refcount
+ * @folio: the folio
+ *
+ * Calculate the expected folio refcount, taking references from the pagecache,
+ * swapcache, PG_private and page table mappings into account. Useful in
+ * combination with folio_ref_count() to detect unexpected references (e.g.,
+ * GUP or other temporary references).
+ *
+ * Does currently not consider references from the LRU cache. If the folio
+ * was isolated from the LRU (which is the case during migration or split),
+ * the LRU cache does not apply.
+ *
+ * Calling this function on an unmapped folio -- !folio_mapped() -- that is
+ * locked will return a stable result.
+ *
+ * Calling this function on a mapped folio will not result in a stable result,
+ * because nothing stops additional page table mappings from coming (e.g.,
+ * fork()) or going (e.g., munmap()).
+ *
+ * Calling this function without the folio lock will also not result in a
+ * stable result: for example, the folio might get dropped from the swapcache
+ * concurrently.
+ *
+ * However, even when called without the folio lock or on a mapped folio,
+ * this function can be used to detect unexpected references early (for example,
+ * if it makes sense to even lock the folio and unmap it).
+ *
+ * The caller must add any reference (e.g., from folio_try_get()) it might be
+ * holding itself to the result.
+ *
+ * Returns the expected folio refcount.
+ */
+static inline int folio_expected_ref_count(const struct folio *folio)
+{
+ const int order = folio_order(folio);
+ int ref_count = 0;
+
+ if (WARN_ON_ONCE(folio_test_slab(folio)))
+ return 0;
+
+ if (folio_test_anon(folio)) {
+ /* One reference per page from the swapcache. */
+ ref_count += folio_test_swapcache(folio) << order;
+ } else if (!((unsigned long)folio->mapping & PAGE_MAPPING_FLAGS)) {
+ /* One reference per page from the pagecache. */
+ ref_count += !!folio->mapping << order;
+ /* One reference from PG_private. */
+ ref_count += folio_test_private(folio);
+ }
+
+ /* One reference per page table mapping. */
+ return ref_count + folio_mapcount(folio);
+}
+
#ifndef HAVE_ARCH_MAKE_FOLIO_ACCESSIBLE
static inline int arch_make_folio_accessible(struct folio *folio)
{
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -445,20 +445,6 @@ unlock:
}
#endif
-static int folio_expected_refs(struct address_space *mapping,
- struct folio *folio)
-{
- int refs = 1;
- if (!mapping)
- return refs;
-
- refs += folio_nr_pages(folio);
- if (folio_test_private(folio))
- refs++;
-
- return refs;
-}
-
/*
* Replace the folio in the mapping.
*
@@ -601,7 +587,7 @@ static int __folio_migrate_mapping(struc
int folio_migrate_mapping(struct address_space *mapping,
struct folio *newfolio, struct folio *folio, int extra_count)
{
- int expected_count = folio_expected_refs(mapping, folio) + extra_count;
+ int expected_count = folio_expected_ref_count(folio) + extra_count + 1;
if (folio_ref_count(folio) != expected_count)
return -EAGAIN;
@@ -618,7 +604,7 @@ int migrate_huge_page_move_mapping(struc
struct folio *dst, struct folio *src)
{
XA_STATE(xas, &mapping->i_pages, folio_index(src));
- int rc, expected_count = folio_expected_refs(mapping, src);
+ int rc, expected_count = folio_expected_ref_count(src) + 1;
if (folio_ref_count(src) != expected_count)
return -EAGAIN;
@@ -749,7 +735,7 @@ static int __migrate_folio(struct addres
struct folio *src, void *src_private,
enum migrate_mode mode)
{
- int rc, expected_count = folio_expected_refs(mapping, src);
+ int rc, expected_count = folio_expected_ref_count(src) + 1;
/* Check whether src does not have extra refs before we do more work */
if (folio_ref_count(src) != expected_count)
@@ -837,7 +823,7 @@ static int __buffer_migrate_folio(struct
return migrate_folio(mapping, dst, src, mode);
/* Check whether page does not have extra refs before we do more work */
- expected_count = folio_expected_refs(mapping, src);
+ expected_count = folio_expected_ref_count(src) + 1;
if (folio_ref_count(src) != expected_count)
return -EAGAIN;
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