[PATCH 1/2] ext4: use fsdata to track inline data write state

[Aditya Srivastava <aditya.ansh182@gmail.com>]

From: Aditya Prakash Srivastava <aditya.ansh182@gmail.com>

Instead of checking the live inode state (`ext4_has_inline_data(inode)`
and `ext4_test_inode_state(inode, EXT4_STATE_MAY_INLINE_DATA)`) in the
write_end handlers, use the `fsdata` parameter of the address space
operations to explicitly pass down the state in which `write_begin`
prepared the write.

A concurrent thread (such as `ext4_page_mkwrite()`) can convert the
inline data to an extent between `write_begin` and `write_end`. If this
happens, the write_end handlers would previously miss the inline
write_end path and fall through to extent-based write_end logic. However,
since block buffers were never allocated in `write_begin`, this resulted
in NULL pointer dereferences or data loss because `folio_buffers(folio)`
was NULL.

By defining `EXT4_WRITE_DATA_INLINE` (3) and communicating this state via
`fsdata`:
1) `ext4_write_begin()` and `ext4_da_write_begin()` explicitly set
   `*fsdata` to `EXT4_WRITE_DATA_INLINE` when an inline write is
   successfully prepared.
2) `ext4_write_end()`, `ext4_journalled_write_end()`, and
   `ext4_da_write_end()` rely solely on `fsdata` / `write_mode` to
   invoke `ext4_write_inline_data_end()`.

This removes the crude race fallbacks and makes the write_end
determination unambiguous, simple, and clean.

Suggested-by: Jan Kara <jack@suse.cz>
Signed-off-by: Aditya Prakash Srivastava <aditya.ansh182@gmail.com>
---
 fs/ext4/ext4.h  |  1 +
 fs/ext4/inode.c | 22 +++++++++++++---------
 2 files changed, 14 insertions(+), 9 deletions(-)

diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
index b37c136ea3ab..521bd5d6321c 100644
--- a/fs/ext4/ext4.h
+++ b/fs/ext4/ext4.h
@@ -3138,6 +3138,7 @@ int do_journal_get_write_access(handle_t *handle, struct inode *inode,
 void ext4_set_inode_mapping_order(struct inode *inode);
 #define FALL_BACK_TO_NONDELALLOC 1
 #define CONVERT_INLINE_DATA	 2
+#define EXT4_WRITE_DATA_INLINE	 3
 
 typedef enum {
 	EXT4_IGET_NORMAL =	0,
diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index ce99807c5f5b..e2e8ac5fb8d8 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -1302,6 +1302,9 @@ static int ext4_write_begin(const struct kiocb *iocb,
 	if (unlikely(ret))
 		return ret;
 
+	if (fsdata && *fsdata != (void *)FALL_BACK_TO_NONDELALLOC)
+		*fsdata = NULL;
+
 	trace_ext4_write_begin(inode, pos, len);
 	/*
 	 * Reserve one block more for addition to orphan list in case
@@ -1316,8 +1319,11 @@ static int ext4_write_begin(const struct kiocb *iocb,
 						    foliop);
 		if (ret < 0)
 			return ret;
-		if (ret == 1)
+		if (ret == 1) {
+			if (fsdata)
+				*fsdata = (void *)EXT4_WRITE_DATA_INLINE;
 			return 0;
+		}
 	}
 
 	/*
@@ -1450,8 +1456,7 @@ static int ext4_write_end(const struct kiocb *iocb,
 
 	trace_ext4_write_end(inode, pos, len, copied);
 
-	if (ext4_has_inline_data(inode) &&
-	    ext4_test_inode_state(inode, EXT4_STATE_MAY_INLINE_DATA))
+	if (fsdata == (void *)EXT4_WRITE_DATA_INLINE)
 		return ext4_write_inline_data_end(inode, pos, len, copied,
 						  folio);
 
@@ -1560,8 +1565,7 @@ static int ext4_journalled_write_end(const struct kiocb *iocb,
 
 	BUG_ON(!ext4_handle_valid(handle));
 
-	if (ext4_has_inline_data(inode) &&
-	    ext4_test_inode_state(inode, EXT4_STATE_MAY_INLINE_DATA))
+	if (fsdata == (void *)EXT4_WRITE_DATA_INLINE)
 		return ext4_write_inline_data_end(inode, pos, len, copied,
 						  folio);
 
@@ -3161,8 +3165,10 @@ static int ext4_da_write_begin(const struct kiocb *iocb,
 						     foliop, fsdata, true);
 		if (ret < 0)
 			return ret;
-		if (ret == 1)
+		if (ret == 1) {
+			*fsdata = (void *)EXT4_WRITE_DATA_INLINE;
 			return 0;
+		}
 	}
 
 retry:
@@ -3299,9 +3305,7 @@ static int ext4_da_write_end(const struct kiocb *iocb,
 
 	trace_ext4_da_write_end(inode, pos, len, copied);
 
-	if (write_mode != CONVERT_INLINE_DATA &&
-	    ext4_test_inode_state(inode, EXT4_STATE_MAY_INLINE_DATA) &&
-	    ext4_has_inline_data(inode))
+	if (write_mode == EXT4_WRITE_DATA_INLINE)
 		return ext4_write_inline_data_end(inode, pos, len, copied,
 						  folio);
 
-- 
2.47.3