Message-ID: <82a096ae-0be2-4f7e-842f-95cfe8df3ba5@amd.com>
Date: Wed, 30 Apr 2025 10:42:34 +0530
From: Shivank Garg <shivankg@....com>
To: linux-kernel@...r.kernel.org,
 syzbot <syzbot+8bb6fd945af4e0ad9299@...kaller.appspotmail.com>
Subject: Syz Test if issue still reproduce

Hi,

Testing if this issue https://syzkaller.appspot.com/bug?extid=8bb6fd945af4e0ad9299
still reproduces


#syz test: git://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git 33035b665

diff --git a/fs/jfs/jfs_metapage.c b/fs/jfs/jfs_metapage.c
index df575a873ec6..f863ba521533 100644
--- a/fs/jfs/jfs_metapage.c
+++ b/fs/jfs/jfs_metapage.c
@@ -15,6 +15,7 @@
 #include <linux/mempool.h>
 #include <linux/seq_file.h>
 #include <linux/writeback.h>
+#include <linux/migrate.h>
 #include "jfs_incore.h"
 #include "jfs_superblock.h"
 #include "jfs_filsys.h"
@@ -151,6 +152,54 @@ static inline void dec_io(struct folio *folio, blk_status_t status,
 		handler(folio, anchor->status);
 }
 
+static int __metapage_migrate_folio(struct address_space *mapping, struct folio *dst,
+				    struct folio *src, enum migrate_mode mode)
+{
+	struct meta_anchor *src_anchor = src->private;
+	struct metapage *mps[MPS_PER_PAGE] = {0};
+	struct metapage *mp;
+	int i, rc;
+
+	for (i = 0; i < MPS_PER_PAGE; i++) {
+		mp = src_anchor->mp[i];
+		if (mp && metapage_locked(mp))
+			return -EAGAIN;
+	}
+
+	rc = filemap_migrate_folio(mapping, dst, src, mode);
+	if (rc != MIGRATEPAGE_SUCCESS)
+		return rc;
+
+	for (i = 0; i < MPS_PER_PAGE; i++) {
+		mp = src_anchor->mp[i];
+		if (!mp)
+			continue;
+		if (unlikely(insert_metapage(dst, mp))) {
+			/* If error, roll-back previosly inserted pages */
+			for (int j = 0 ; j < i; j++) {
+				if (mps[j])
+					remove_metapage(dst, mps[j]);
+			}
+			return -EAGAIN;
+		}
+		mps[i] = mp;
+	}
+
+	/* Update the metapage and remove it from src */
+	for (i = 0; i < MPS_PER_PAGE; i++) {
+		mp = mps[i];
+		if (mp) {
+			int page_offset = mp->data - folio_address(src);
+
+			mp->data = folio_address(dst) + page_offset;
+			mp->folio = dst;
+			remove_metapage(src, mp);
+		}
+	}
+
+	return MIGRATEPAGE_SUCCESS;
+}
+
 #else
 static inline struct metapage *folio_to_mp(struct folio *folio, int offset)
 {
@@ -175,6 +224,32 @@ static inline void remove_metapage(struct folio *folio, struct metapage *mp)
 #define inc_io(folio) do {} while(0)
 #define dec_io(folio, status, handler) handler(folio, status)
 
+static int __metapage_migrate_folio(struct address_space *mapping, struct folio *dst,
+				    struct folio *src, enum migrate_mode mode)
+{
+	struct metapage *mp;
+	int page_offset;
+	int rc;
+
+	mp = folio_to_mp(src, 0);
+	if (mp && metapage_locked(mp))
+		return -EAGAIN;
+
+	rc = filemap_migrate_folio(mapping, dst, src, mode);
+	if (rc != MIGRATEPAGE_SUCCESS)
+		return rc;
+
+	if (unlikely(insert_metapage(dst, mp)))
+		return -EAGAIN;
+
+	page_offset = mp->data - folio_address(src);
+	mp->data = folio_address(dst) + page_offset;
+	mp->folio = dst;
+	remove_metapage(src, mp);
+
+	return MIGRATEPAGE_SUCCESS;
+}
+
 #endif
 
 static inline struct metapage *alloc_metapage(gfp_t gfp_mask)
@@ -554,6 +629,24 @@ static bool metapage_release_folio(struct folio *folio, gfp_t gfp_mask)
 	return ret;
 }
 
+/**
+ * metapage_migrate_folio - Migration function for JFS metapages
+ */
+static int metapage_migrate_folio(struct address_space *mapping, struct folio *dst,
+				  struct folio *src, enum migrate_mode mode)
+{
+	int expected_count;
+
+	if (!src->private)
+		return filemap_migrate_folio(mapping, dst, src, mode);
+
+	/* Check whether page does not have extra refs before we do more work */
+	expected_count = folio_expected_ref_count(src) + 1;
+	if (folio_ref_count(src) != expected_count)
+		return -EAGAIN;
+	return __metapage_migrate_folio(mapping, dst, src, mode);
+}
+
 static void metapage_invalidate_folio(struct folio *folio, size_t offset,
 				    size_t length)
 {
@@ -570,6 +663,7 @@ const struct address_space_operations jfs_metapage_aops = {
 	.release_folio	= metapage_release_folio,
 	.invalidate_folio = metapage_invalidate_folio,
 	.dirty_folio	= filemap_dirty_folio,
+	.migrate_folio	= metapage_migrate_folio,
 };
 
 struct metapage *__get_metapage(struct inode *inode, unsigned long lblock,
diff --git a/include/linux/mm.h b/include/linux/mm.h
index 68d3dd14b323..1db540bb9381 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -2115,6 +2115,61 @@ static inline bool folio_maybe_mapped_shared(struct folio *folio)
 	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)
 {
diff --git a/mm/migrate.c b/mm/migrate.c
index c80591514e66..67f2bf1eb51e 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -445,20 +445,6 @@ void pmd_migration_entry_wait(struct mm_struct *mm, pmd_t *pmd)
 }
 #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(struct address_space *mapping,
 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(struct address_space *mapping,
 				   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 address_space *mapping, struct folio *dst,
 			   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 address_space *mapping,
 		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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