Re: [PATCH v3] ext4: fix race in ext4_convert_inline_data() leading to BUG_ON in writepages

["Zhou, Yun" <yun.zhou@windriver.com>]

Please ignore this patch. I have fixed the issue flagged by Sashiko AI, 
and the updated version will be sent out shortly.

On 7/3/2026 11:23 AM, Yun Zhou wrote:
> ext4_convert_inline_data() has two unsafe lockless fast paths that can
> observe a transient state from a concurrent convert_inline_data_nolock()
> that has destroyed inline data but then fails and restores it.  A racing
> thread sees the transient state, concludes no conversion is needed, and
> creates dirty pages via block_page_mkwrite().  The restoring thread then
> re-sets inline data + MAY_INLINE_DATA, leaving dirty pages coexisting
> with inline data, triggering BUG_ON in ext4_do_writepages().
> 
> A similar race exists with ext4_da_convert_inline_data_to_extent():
> after it copies data to page cache and clears MAY_INLINE_DATA, a
> concurrent convert_inline_data_nolock() can restore MAY_INLINE_DATA.
> 
> Simply moving the MAY_INLINE_DATA check inside xattr_sem would require
> taking the write lock, starting a journal handle, and reading the inode
> location on every call -- even for inodes that have long since been
> converted.  This causes measurable performance regression on filesystems
> with the inline_data feature enabled.
> 
> Fix this by introducing EXT4_STATE_INLINE_CONVERTED, a monotonic
> (set once, never cleared) per-inode state bit indicating that inline
> data has been successfully copied out to page cache or a data block.
> This provides:
> 
> 1. A safe lockless fast path in ext4_convert_inline_data(): once the
>     bit is set, return 0 immediately without any locking overhead.
> 
> 2. A WARN_ON_ONCE guard in convert_inline_data_nolock(): if the bit
>     is unexpectedly set at restore time, warn.  This should not happen
>     given the MAY_INLINE_DATA check prevents entering nolock when DA
>     conversion is in progress.
> 
> 3. Inside xattr_sem, only call convert_inline_data_nolock() when both
>     has_inline_data AND MAY_INLINE_DATA are set.  When has_inline_data
>     is set but MAY_INLINE_DATA is clear, DA conversion is in progress;
>     skip convert_nolock to avoid destroy+restore re-setting MAY.
> 
> The bit is set in all successful conversion paths:
> - ext4_convert_inline_data_nolock() on success
> - ext4_convert_inline_data() when inline data is gone after nolock
> - ext4_da_convert_inline_data_to_extent() after page cache copy
> - ext4_convert_inline_data_to_extent() after block_commit_write
> 
> Fixes: 7b4cc9787fe3 ("ext4: evict inline data when writing to memory map")
> Reported-by: syzbot+d1da16f03614058fdc48@syzkaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=d1da16f03614058fdc48
> Signed-off-by: Yun Zhou <yun.zhou@windriver.com>
> ---
> v3:
> - Introduce EXT4_STATE_INLINE_CONVERTED monotonic bit for safe lockless
>    fast path, avoiding performance regression from always taking xattr_sem.
> - Add WARN_ON_ONCE at restore point as sanity check.
> - Skip convert_nolock when MAY_INLINE_DATA is clear (DA convert in
>    progress) to prevent destroy+restore re-setting MAY.
> 
> v2:
> - Reworked fix: root cause confirmed via ftrace.
> - Removed both unsafe lockless fast paths, serialize under xattr_sem.
> - Skip convert_nolock when MAY_INLINE_DATA is clear (DA convert in
>    progress) to prevent destroy+restore from re-setting MAY.
> 
>   fs/ext4/ext4.h   |  1 +
>   fs/ext4/inline.c | 44 ++++++++++++++++++++++++++------------------
>   2 files changed, 27 insertions(+), 18 deletions(-)
> 
> diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
> index b9b0ada7774b..310aeb14f9f0 100644
> --- a/fs/ext4/ext4.h
> +++ b/fs/ext4/ext4.h
> @@ -2049,6 +2049,7 @@ enum {
>   	EXT4_STATE_FC_FLUSHING_DATA,	/* Fast commit flushing data */
>   	EXT4_STATE_ORPHAN_FILE,		/* Inode orphaned in orphan file */
>   	EXT4_STATE_FC_REQUEUE,		/* Inode modified during fast commit */
> +	EXT4_STATE_INLINE_CONVERTED,	/* inline data copied out, do not restore */
>   };
>   
>   #define EXT4_INODE_BIT_FNS(name, field, offset)				\
> diff --git a/fs/ext4/inline.c b/fs/ext4/inline.c
> index 8045e4ff270c..1b6abacf12e6 100644
> --- a/fs/ext4/inline.c
> +++ b/fs/ext4/inline.c
> @@ -671,6 +671,8 @@ static int ext4_convert_inline_data_to_extent(struct address_space *mapping,
>   
>   	if (folio)
>   		block_commit_write(folio, from, to);
> +	if (folio && !ret)
> +		ext4_set_inode_state(inode, EXT4_STATE_INLINE_CONVERTED);
>   out:
>   	if (folio) {
>   		folio_unlock(folio);
> @@ -921,6 +923,7 @@ static int ext4_da_convert_inline_data_to_extent(struct address_space *mapping,
>   	clear_buffer_new(folio_buffers(folio));
>   	folio_mark_dirty(folio);
>   	folio_mark_uptodate(folio);
> +	ext4_set_inode_state(inode, EXT4_STATE_INLINE_CONVERTED);
>   	ext4_clear_inode_state(inode, EXT4_STATE_MAY_INLINE_DATA);
>   	*fsdata = (void *)CONVERT_INLINE_DATA;
>   
> @@ -1172,8 +1175,14 @@ static int ext4_convert_inline_data_nolock(handle_t *handle,
>   	}
>   
>   out_restore:
> -	if (error)
> -		ext4_restore_inline_data(handle, inode, iloc, buf, inline_size);
> +	if (error) {
> +		WARN_ON_ONCE(ext4_test_inode_state(inode,
> +					EXT4_STATE_INLINE_CONVERTED));
> +		ext4_restore_inline_data(handle, inode, iloc, buf,
> +					 inline_size);
> +	} else {
> +		ext4_set_inode_state(inode, EXT4_STATE_INLINE_CONVERTED);
> +	}
>   
>   out:
>   	brelse(data_bh);
> @@ -1959,22 +1968,16 @@ int ext4_convert_inline_data(struct inode *inode)
>   	handle_t *handle;
>   	struct ext4_iloc iloc;
>   
> -	if (!ext4_has_inline_data(inode)) {
> -		ext4_clear_inode_state(inode, EXT4_STATE_MAY_INLINE_DATA);
> +	if (!ext4_has_feature_inline_data(inode->i_sb))
> +		return 0;
> +
> +	/*
> +	 * Once inline data has been successfully copied out (to page
> +	 * cache or a data block), this bit is set and never cleared.
> +	 * It is safe to check without locks -- the bit is monotonic.
> +	 */
> +	if (ext4_test_inode_state(inode, EXT4_STATE_INLINE_CONVERTED))
>   		return 0;
> -	} else if (!ext4_test_inode_state(inode, EXT4_STATE_MAY_INLINE_DATA)) {
> -		/*
> -		 * Inode has inline data but EXT4_STATE_MAY_INLINE_DATA is
> -		 * cleared. This means we are in the middle of moving of
> -		 * inline data to delay allocated block. Just force writeout
> -		 * here to finish conversion.
> -		 */
> -		error = filemap_flush(inode->i_mapping);
> -		if (error)
> -			return error;
> -		if (!ext4_has_inline_data(inode))
> -			return 0;
> -	}
>   
>   	needed_blocks = ext4_chunk_trans_extent(inode, 1);
>   
> @@ -1990,8 +1993,13 @@ int ext4_convert_inline_data(struct inode *inode)
>   	}
>   
>   	ext4_write_lock_xattr(inode, &no_expand);
> -	if (ext4_has_inline_data(inode))
> +	if (ext4_has_inline_data(inode) &&
> +	    ext4_test_inode_state(inode, EXT4_STATE_MAY_INLINE_DATA))
>   		error = ext4_convert_inline_data_nolock(handle, inode, &iloc);
> +	if (!ext4_has_inline_data(inode)) {
> +		ext4_set_inode_state(inode, EXT4_STATE_INLINE_CONVERTED);
> +		ext4_clear_inode_state(inode, EXT4_STATE_MAY_INLINE_DATA);
> +	}
>   	ext4_write_unlock_xattr(inode, &no_expand);
>   	ext4_journal_stop(handle);
>   out_free: