jbd2: store jinode dirty range in PAGE_SIZE units

jbd2_inode fields are updated under journal->j_list_lock, but some paths
read them without holding the lock (e.g. fast commit helpers and ordered
truncate helpers).

READ_ONCE() alone is not sufficient for the dirty range fields when they
are stored as loff_t because 32-bit platforms can observe torn loads.
Store the dirty range in PAGE_SIZE units as pgoff_t instead.

Represent the dirty range end as an exclusive end page. This avoids a
special sentinel value and keeps MAX_LFS_FILESIZE on 32-bit representable.

Publish a new dirty range by updating end_page before start_page, and
treat start_page >= end_page as empty in the accessor for robustness.

Use READ_ONCE() on the read side and WRITE_ONCE() on the write side for the
dirty range and i_flags to match the existing lockless access pattern.

Suggested-by: Jan Kara <jack@suse.cz>
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Li Chen <me@linux.beauty>
Link: https://patch.msgid.link/20260306085643.465275-5-me@linux.beauty
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

1 files changed, 22 insertions, 12 deletions

diff --git a/include/linux/jbd2.h b/include/linux/jbd2.h
index 64392baf5f4b..7e785aa6d35d 100644
--- a/include/linux/jbd2.h
+++ b/include/linux/jbd2.h
@@ -429,33 +429,43 @@ struct jbd2_inode {
 	unsigned long i_flags;
 
 	/**
-	 * @i_dirty_start:
+	 * @i_dirty_start_page:
+	 *
+	 * Dirty range start in PAGE_SIZE units.
+	 *
+	 * The dirty range is empty if @i_dirty_start_page is greater than or
+	 * equal to @i_dirty_end_page.
 	 *
-	 * Offset in bytes where the dirty range for this inode starts.
 	 * [j_list_lock]
 	 */
-	loff_t i_dirty_start;
+	pgoff_t i_dirty_start_page;
 
 	/**
-	 * @i_dirty_end:
+	 * @i_dirty_end_page:
+	 *
+	 * Dirty range end in PAGE_SIZE units (exclusive).
 	 *
-	 * Inclusive offset in bytes where the dirty range for this inode
-	 * ends. [j_list_lock]
+	 * [j_list_lock]
 	 */
-	loff_t i_dirty_end;
+	pgoff_t i_dirty_end_page;
 };
 
+/*
+ * Lockless readers treat start_page >= end_page as an empty range.
+ * Writers publish a new non-empty range by storing i_dirty_end_page before
+ * i_dirty_start_page.
+ */
 static inline bool jbd2_jinode_get_dirty_range(const struct jbd2_inode *jinode,
 					       loff_t *start, loff_t *end)
 {
-	loff_t start_byte = jinode->i_dirty_start;
-	loff_t end_byte = jinode->i_dirty_end;
+	pgoff_t start_page = READ_ONCE(jinode->i_dirty_start_page);
+	pgoff_t end_page = READ_ONCE(jinode->i_dirty_end_page);
 
-	if (!end_byte)
+	if (start_page >= end_page)
 		return false;
 
-	*start = start_byte;
-	*end = end_byte;
+	*start = (loff_t)start_page << PAGE_SHIFT;
+	*end = ((loff_t)end_page << PAGE_SHIFT) - 1;
 	return true;
 }