Re: [PATCH v2 1/6] ext4: move partial block zeroing earlier in ext4_zero_range()

[Zhang Yi]

On 7/7/2026 10:13 AM, Zhang Yi wrote:
> In ext4_zero_range(), move the ext4_zero_partial_blocks() call, which
> handles unaligned edges, into the same branch where the unaligned range
> is preallocated, immediately after ext4_alloc_file_blocks(). This is
> safe because there is no dependency between partial block handling and
> the subsequent full block handling.
> 
> This change will be used by later patches that handle unaligned
> FALLOC_FL_WRITE_ZEROES operations, which will need to check the partial
> zeroed result.
> 
> Signed-off-by: Zhang Yi <yi.zhang@xxxxxxxxxx>
> Reviewed-by: Jan Kara <jack@xxxxxxx>
> ---
>  fs/ext4/extents.c | 12 +++++++-----
>  1 file changed, 7 insertions(+), 5 deletions(-)
> 
> diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
> index 125f628e738a..27ca641e701b 100644
> --- a/fs/ext4/extents.c
> +++ b/fs/ext4/extents.c
> @@ -4734,10 +4734,16 @@ static long ext4_zero_range(struct file *file, loff_t offset,
>  	}
>  
>  	flags = EXT4_GET_BLOCKS_CREATE_UNWRIT_EXT;
> -	/* Preallocate the range including the unaligned edges */
> +	/*
> +	 * Preallocate the range including the unaligned edges, and zero
> +	 * out partial blocks if they already contain data.
> +	 */
>  	if (!IS_ALIGNED(offset | end, blocksize)) {
>  		ret = ext4_alloc_file_blocks(file, offset, len, new_size,
>  					     flags);
> +		if (!ret)
> +			ret = ext4_zero_partial_blocks(inode, offset, len,
> +						       &partial_zeroed);
>  		if (ret)
>  			return ret;

Sashiko pointed out below:

>Is it safe to modify these folios before calling
>ext4_truncate_page_cache_block_range()?

>Previously, ext4_zero_range() called ext4_truncate_page_cache_block_range()
>first, which waits for writeback on the boundary folios via
>truncate_pagecache_range() -> truncate_inode_pages_range() ->
>truncate_inode_partial_folio().

>By zeroing the partial blocks earlier, ext4_block_do_zero_range() might
>acquire the folio lock and mutate the folio data while PG_writeback is still
>set, potentially causing data corruption or checksum mismatches if the block
>layer is actively DMA-ing it to disk.

I think this issue is confined to stable devices. The proposed fix is
to modify the fgf_flags passed to the folio lookup function in
ext4_load_tail_bh() by setting them to FGP_WRITEBEGIN | FGP_ACCESSED.

Thanks,
Yi.

>  	}
> @@ -4770,10 +4776,6 @@ static long ext4_zero_range(struct file *file, loff_t offset,
>  	if (IS_ALIGNED(offset | end, blocksize))
>  		return ret;
>  
> -	/* Zero out partial block at the edges of the range */
> -	ret = ext4_zero_partial_blocks(inode, offset, len, &partial_zeroed);
> -	if (ret)
> -		return ret;
>  	if (((file->f_flags & O_SYNC) || IS_SYNC(inode)) && partial_zeroed) {
>  		ret = filemap_write_and_wait_range(inode->i_mapping, offset,
>  						   end - 1);