FAILED: patch "[PATCH] btrfs: do not overwrite NODATASUM flag when removing" failed to apply to 6.1-stable tree

The patch below does not apply to the 6.1-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.1.y
git checkout FETCH_HEAD
git cherry-pick -x 15f7c86215e8d5f14b24127fa88af6c79363d50e
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2026090330-antelope-bullfrog-bfbf@gregkh' --subject-prefix 'PATCH 6.1.y' 'HEAD^..'

Possible dependencies:

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 15f7c86215e8d5f14b24127fa88af6c79363d50e Mon Sep 17 00:00:00 2001
From: Qu Wenruo <[email protected]>
Date: Tue, 9 Jun 2026 08:43:34 +0930
Subject: [PATCH] btrfs: do not overwrite NODATASUM flag when removing
NODATACOW flag

[TEST FAILURE]
The test case generic/628 will fail if MOUNT_OPTIONS is set to "-o nodatasum":

  FSTYP         -- btrfs
  PLATFORM      -- Linux/x86_64 btrfs-vm 7.1.0-rc4-custom+ #383 SMP PREEMPT_DYNAMIC Sat May 30 07:35:42 ACST 2026
  MKFS_OPTIONS  -- -O bgt -K /dev/mapper/test-scratch1

MOUNT_OPTIONS -- -o nodatasum /dev/mapper/test-scratch1 /mnt/scratch

generic/628 1s ... - output mismatch (see /home/adam/xfstests/results//generic/628.out.bad)

     --- tests/generic/628.out	2022-05-11 11:25:30.816666664 +0930
     +++ /home/adam/xfstests/results//generic/628.out.bad	2026-06-08 18:56:49.878542927 +0930
     @@ -8,8 +8,9 @@
      310f146ce52077fcd3308dcbe7632bb2  SCRATCH_MNT/a
      310f146ce52077fcd3308dcbe7632bb2  SCRATCH_MNT/d
      test reflink flag not set iflag
     +XFS_IOC_CLONE: Invalid argument
      310f146ce52077fcd3308dcbe7632bb2  SCRATCH_MNT/a
     -310f146ce52077fcd3308dcbe7632bb2  SCRATCH_MNT/b
     +d41d8cd98f00b204e9800998ecf8427e  SCRATCH_MNT/b
     ...

[CAUSE]
The direct cause is that after "chattr +S", the btrfs inode will lose its NODATASUM flag inherited from the mount option. E.g.:

  # mkfs.btrfs -f $dev
  # mount $dev $mnt -o nodatasum
  # touch $mnt/foobar
  # sync
  # btrfs ins dump-tree -t 5 $dev | grep "(257 INODE_ITEM 0) itemoff" -A 3
         item 4 key (257 INODE_ITEM 0) itemoff 15879 itemsize 160
         	generation 9 transid 9 size 0 nbytes 0
         	block group 0 mode 100644 links 1 uid 0 gid 0 rdev 0
         	sequence 1 flags 0x1(NODATASUM)
         	                     ^^^^^^^^^ Proper NODATASUM flag

  # chattr +S $mnt/foobar
  # sync
  # btrfs ins dump-tree -t 5 $dev | grep "(257 INODE_ITEM 0) itemoff" -A 3
  	item 4 key (257 INODE_ITEM 0) itemoff 15879 itemsize 160
         	generation 9 transid 10 size 0 nbytes 0
         	block group 0 mode 100644 links 1 uid 0 gid 0 rdev 0
         	sequence 2 flags 0x20(SYNC)
         	                      ^^^^ Only the new SYNC flag

This makes the inode drop the old NODATASUM flag, while the new reflink destination will still inherit the NODATASUM flag. The mismatching NODATASUM flags will cause the reflink to fail.

The root cause is that, inside btrfs_fileattr_set() if no FS_NOCOW_FL is set, we remove both NODATASUM and NODATACOW flag.

However we should not touch NODATASUM flag, as data COW doesn't require checksum. Only NODATACOW implies NODATASUM, but DATACOW doesn't imply DATASUM.

The deeper problems are:

- Fileattr API is too binary
It either clears or sets a flag, there is no "do not change" option. So that why "chattr +S" implies "chattr -C", and is forcing us to change NODATACOW along with NODATASUM flag.

- No way to change NODATASUM through fileattr API
In fact NODATASUM can only be modified through mount option.

The deeper problems are much harder to attack.

[FIX]
Remove NODATACOW flag when FS_NOCOW_FL is not set, but only remove NODATASUM if "nodatasum" mount option is not set.

This allows the existing "chattr +C" then "chattr -C" to remove both NODATACOW and NODATASUM flags on a default mount.

But for a mount with "nodatasum" option, the NODATASUM inode flag will persist through either "chattr +C" and "chattr -C".

index 7e97a6aebf9b..32dd7bbd4d63 100644
@@ -356,14 +356,21 @@ int btrfs_fileattr_set(struct mnt_idmap *idmap,
 			inode_flags |= BTRFS_INODE_NODATACOW;
 		}
 	} else {
-		/*
-		 * Revert back under same assumptions as above
-		 */
-		if (S_ISREG(inode->vfs_inode.i_mode)) {
-			if (inode->vfs_inode.i_size == 0)
-				inode_flags &= ~(BTRFS_INODE_NODATACOW |
-						 BTRFS_INODE_NODATASUM);
-		} else {
+		/* We can only change NODATACOW for zero-sized regular file. */
+		if (S_ISREG(inode->vfs_inode.i_mode) && (inode->vfs_inode.i_size == 0)) {
+			inode_flags &= ~BTRFS_INODE_NODATACOW;
+			/*
+			 * There is currently no way to change NODATASUM flag
+			 * through fileattr API.  If we unconditionally keep the
+			 * current NODATASUM flag, chattr +C then chattr -C will
+			 * keep the NODATASUM flag, and no way to remove that
+			 * flag.
+			 *
+			 * So respect the current mount option for NODATASUM flag.
+			 */
+			if (!btrfs_test_opt(fs_info, NODATASUM))
+				inode_flags &= ~BTRFS_INODE_NODATASUM;
+		} else if (!S_ISREG(inode->vfs_inode.i_mode)) {
 			inode_flags &= ~BTRFS_INODE_NODATACOW;
 		}
 	}

From: Filipe Manana <[email protected]>

[ Upstream commit a4545b74e2de98989c1d14bc48c52c2f9fa734b6 ]

Currently BTRFS_BYTES_TO_BLKS() is defined in ctree.h but it's not related at all to the btree data structure, so move it into fs.h.

[ sashal: Reduced backport -- upstream a4545b74e2de9 touches 2 file(s), this
backport carries 2. Not backported here:
fs/btrfs/fs.h
This note is generated from the file lists only; see the resolution record for the reasoning. ]

Stable-dep-of: 15f7c86215e8 ("btrfs: do not overwrite NODATASUM flag when removing NODATACOW flag")

---
 fs/btrfs/ctree.h |  6 ++--
 fs/btrfs/ioctl.c | 85 ++++++++++++++++++++++++------------------------
 2 files changed, 45 insertions(+), 46 deletions(-)

index 16795e91aee29..c2482c2b634cd 100644
@@ -1130,6 +1130,9 @@ static inline u64 btrfs_calc_metadata_size(struct btrfs_fs_info *fs_info,
 #define BTRFS_MAX_EXTENT_ITEM_SIZE(r) ((BTRFS_LEAF_DATA_SIZE(r->fs_info) >> 4) - \
 					sizeof(struct btrfs_item))
 
+#define BTRFS_BYTES_TO_BLKS(fs_info, bytes) \
+				((bytes) >> (fs_info)->sectorsize_bits)
+
 static inline bool btrfs_is_zoned(const struct btrfs_fs_info *fs_info)
 {
 	return fs_info->zone_size > 0;

@@ -1733,9 +1736,6 @@ struct btrfs_map_token {
 	unsigned long offset;
 };
 
-#define BTRFS_BYTES_TO_BLKS(fs_info, bytes) \
-				((bytes) >> (fs_info)->sectorsize_bits)
-
 static inline void btrfs_init_map_token(struct btrfs_map_token *token,
 					struct extent_buffer *eb)
 {

2.53.0

From: Qu Wenruo <[email protected]>

[ Upstream commit ecde48a1a6b3256bd49db8780bf37556b157783c ]

The address space flag AS_STABLE_WRITES determine if FGP_STABLE for will wait for the folio to finish its writeback.

For btrfs, due to the default data checksum behavior, if we modify the folio while it's still under writeback, it will cause data checksum mismatch. Thus for quite some call sites we manually call folio_wait_writeback() to prevent such problem from happening.

Currently there is only one call site inside btrfs really utilizing FGP_STABLE, and in that case we also manually call folio_wait_writeback() to do the waiting.

But it's better to properly expose the stable writes flag to a per-inode basis, to allow call sites to fully benefit from FGP_STABLE flag. E.g. for inodes with NODATASUM allowing beginning dirtying the page without waiting for writeback.

This involves:

- Update the mapping's stable write flag when setting/clearing NODATASUM inode flag using ioctl
This only works for empty files, so it should be fine.

- Update the mapping's stable write flag when reading an inode from disk

- Remove the explicit folio_wait_writeback() for FGP_BEGINWRITE call site

[ sashal: Reduced backport -- upstream ecde48a1a6b32 touches 4 file(s), this
backport carries 3. Not backported here:
fs/btrfs/btrfs_inode.h
This note is generated from the file lists only; see the resolution record for the reasoning. ]

Stable-dep-of: 15f7c86215e8 ("btrfs: do not overwrite NODATASUM flag when removing NODATACOW flag")

---
 fs/btrfs/file.c  | 3 +--
 fs/btrfs/inode.c | 8 ++++++++
 fs/btrfs/ioctl.c | 4 ++++
 3 files changed, 13 insertions(+), 2 deletions(-)

index 0970ec776ff8e..a0224ea15e0fd 100644
@@ -863,7 +863,7 @@ static int prepare_uptodate_page(struct inode *inode,
 
 static unsigned int get_prepare_fgp_flags(bool nowait)
 {
-	unsigned int fgp_flags = FGP_LOCK | FGP_ACCESSED | FGP_CREAT;
+	unsigned int fgp_flags = FGP_LOCK | FGP_ACCESSED | FGP_CREAT | FGP_STABLE;
 
 	if (nowait)
 		fgp_flags |= FGP_NOWAIT;

@@ -933,7 +933,6 @@ static noinline int prepare_pages(struct inode *inode, struct page **pages,
 			faili = i - 1;
 			goto fail;
 		}
-		wait_on_page_writeback(pages[i]);
 	}
 
 	return 0;

index b2a1d96a806a9..8e34328fa5da5 100644
@@ -4092,6 +4092,10 @@ static int btrfs_read_locked_inode(struct inode *inode,
 	BTRFS_I(inode)->index_cnt = (u64)-1;
 	btrfs_inode_split_flags(btrfs_inode_flags(leaf, inode_item),
 				&BTRFS_I(inode)->flags, &BTRFS_I(inode)->ro_flags);
+	if (BTRFS_I(inode)->flags & BTRFS_INODE_NODATASUM)
+		mapping_clear_stable_writes(inode->i_mapping);
+	else
+		mapping_set_stable_writes(inode->i_mapping);
 
 cache_index:
 	/*

@@ -6582,6 +6586,10 @@ int btrfs_create_new_inode(struct btrfs_trans_handle *trans,
 		if (btrfs_test_opt(fs_info, NODATACOW))
 			BTRFS_I(inode)->flags |= BTRFS_INODE_NODATACOW |
 				BTRFS_INODE_NODATASUM;
+		if (BTRFS_I(inode)->flags & BTRFS_INODE_NODATASUM)
+			mapping_clear_stable_writes(inode->i_mapping);
+		else
+			mapping_set_stable_writes(inode->i_mapping);
 	}
 
 	location = &BTRFS_I(inode)->location;

index 6dc4e88827740..6c003682f3650 100644
@@ -370,6 +370,10 @@ int btrfs_fileattr_set(struct user_namespace *mnt_userns,
 
 update_flags:
 	inode->flags = inode_flags;
+	if (inode->flags & BTRFS_INODE_NODATASUM)
+		mapping_clear_stable_writes(inode->vfs_inode.i_mapping);
+	else
+		mapping_set_stable_writes(inode->vfs_inode.i_mapping);
 	btrfs_sync_inode_flags_to_i_flags(&inode->vfs_inode);
 	inode_inc_iversion(&inode->vfs_inode);
 	inode->vfs_inode.i_ctime = current_time(&inode->vfs_inode);

2.53.0

From: Qu Wenruo <[email protected]>

[ Upstream commit 15f7c86215e8d5f14b24127fa88af6c79363d50e ]

[TEST FAILURE]
The test case generic/628 will fail if MOUNT_OPTIONS is set to "-o nodatasum":

  FSTYP         -- btrfs
  PLATFORM      -- Linux/x86_64 btrfs-vm 7.1.0-rc4-custom+ #383 SMP PREEMPT_DYNAMIC Sat May 30 07:35:42 ACST 2026
  MKFS_OPTIONS  -- -O bgt -K /dev/mapper/test-scratch1

MOUNT_OPTIONS -- -o nodatasum /dev/mapper/test-scratch1 /mnt/scratch

generic/628 1s ... - output mismatch (see /home/adam/xfstests/results//generic/628.out.bad)

     --- tests/generic/628.out	2022-05-11 11:25:30.816666664 +0930
     +++ /home/adam/xfstests/results//generic/628.out.bad	2026-06-08 18:56:49.878542927 +0930
     @@ -8,8 +8,9 @@
      310f146ce52077fcd3308dcbe7632bb2  SCRATCH_MNT/a
      310f146ce52077fcd3308dcbe7632bb2  SCRATCH_MNT/d
      test reflink flag not set iflag
     +XFS_IOC_CLONE: Invalid argument
      310f146ce52077fcd3308dcbe7632bb2  SCRATCH_MNT/a
     -310f146ce52077fcd3308dcbe7632bb2  SCRATCH_MNT/b
     +d41d8cd98f00b204e9800998ecf8427e  SCRATCH_MNT/b
     ...

[CAUSE]
The direct cause is that after "chattr +S", the btrfs inode will lose its NODATASUM flag inherited from the mount option. E.g.:

  # mkfs.btrfs -f $dev
  # mount $dev $mnt -o nodatasum
  # touch $mnt/foobar
  # sync
  # btrfs ins dump-tree -t 5 $dev | grep "(257 INODE_ITEM 0) itemoff" -A 3
         item 4 key (257 INODE_ITEM 0) itemoff 15879 itemsize 160
         	generation 9 transid 9 size 0 nbytes 0
         	block group 0 mode 100644 links 1 uid 0 gid 0 rdev 0
         	sequence 1 flags 0x1(NODATASUM)
         	                     ^^^^^^^^^ Proper NODATASUM flag

  # chattr +S $mnt/foobar
  # sync
  # btrfs ins dump-tree -t 5 $dev | grep "(257 INODE_ITEM 0) itemoff" -A 3
  	item 4 key (257 INODE_ITEM 0) itemoff 15879 itemsize 160
         	generation 9 transid 10 size 0 nbytes 0
         	block group 0 mode 100644 links 1 uid 0 gid 0 rdev 0
         	sequence 2 flags 0x20(SYNC)
         	                      ^^^^ Only the new SYNC flag

This makes the inode drop the old NODATASUM flag, while the new reflink destination will still inherit the NODATASUM flag. The mismatching NODATASUM flags will cause the reflink to fail.

The root cause is that, inside btrfs_fileattr_set() if no FS_NOCOW_FL is set, we remove both NODATASUM and NODATACOW flag.

However we should not touch NODATASUM flag, as data COW doesn't require checksum. Only NODATACOW implies NODATASUM, but DATACOW doesn't imply DATASUM.

The deeper problems are:

- Fileattr API is too binary
It either clears or sets a flag, there is no "do not change" option. So that why "chattr +S" implies "chattr -C", and is forcing us to change NODATACOW along with NODATASUM flag.

- No way to change NODATASUM through fileattr API
In fact NODATASUM can only be modified through mount option.

The deeper problems are much harder to attack.

[FIX]
Remove NODATACOW flag when FS_NOCOW_FL is not set, but only remove NODATASUM if "nodatasum" mount option is not set.

This allows the existing "chattr +C" then "chattr -C" to remove both NODATACOW and NODATASUM flags on a default mount.

But for a mount with "nodatasum" option, the NODATASUM inode flag will persist through either "chattr +C" and "chattr -C".

---
 fs/btrfs/ioctl.c | 23 +++++++++++++++--------
 1 file changed, 15 insertions(+), 8 deletions(-)

index 6c003682f3650..7c56360363409 100644
@@ -309,14 +309,21 @@ int btrfs_fileattr_set(struct user_namespace *mnt_userns,
 			inode_flags |= BTRFS_INODE_NODATACOW;
 		}
 	} else {
-		/*
-		 * Revert back under same assumptions as above
-		 */
-		if (S_ISREG(inode->vfs_inode.i_mode)) {
-			if (inode->vfs_inode.i_size == 0)
-				inode_flags &= ~(BTRFS_INODE_NODATACOW |
-						 BTRFS_INODE_NODATASUM);
-		} else {
+		/* We can only change NODATACOW for zero-sized regular file. */
+		if (S_ISREG(inode->vfs_inode.i_mode) && (inode->vfs_inode.i_size == 0)) {
+			inode_flags &= ~BTRFS_INODE_NODATACOW;
+			/*
+			 * There is currently no way to change NODATASUM flag
+			 * through fileattr API.  If we unconditionally keep the
+			 * current NODATASUM flag, chattr +C then chattr -C will
+			 * keep the NODATASUM flag, and no way to remove that
+			 * flag.
+			 *
+			 * So respect the current mount option for NODATASUM flag.
+			 */
+			if (!btrfs_test_opt(fs_info, NODATASUM))
+				inode_flags &= ~BTRFS_INODE_NODATASUM;
+		} else if (!S_ISREG(inode->vfs_inode.i_mode)) {
 			inode_flags &= ~BTRFS_INODE_NODATACOW;
 		}
 	}

2.53.0