Re: [PATCH] ext4: do not advance i_size on a buffered write that copied nothing

[Zhang Yi]

On 10/9/2026 12:46 PM, Hengyu Liang wrote:
> Commit 665575cff098 ("filemap: move prefaulting out of hot write path")
> made generic_perform_write() call ->write_begin() and ->write_end()
> before it finds out that the source buffer cannot be read.  In that
> case ->write_end() is called with copied == 0.
> 
> However, ext4_write_end(), ext4_journalled_write_end() and
> ext4_da_do_write_end() set i_size to pos + copied without checking
> copied.  As of now, a buffered write() past EOF that fails with EFAULT
> grows the file to the write offset, and the new size can reach the
> disk.
> 
> The issue can be reproduced on an ext4 mount with:
> 
>     char *p = mmap(NULL, 4096, PROT_NONE,
>                    MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
>     int fd = open("/mnt/ext4/file", O_CREAT | O_TRUNC | O_RDWR, 0600);
>     pwrite(fd, p, 4096, 1024 * 1024);   /* fails with EFAULT */
>     fstat(fd, &st);
> 
> Before that commit, the result is st_size = 0.
> After it, the result is st_size = 1048576.
> 
> This patch will leave i_size alone when nothing was copied, as
> ext4_write_inline_data_end() already does.  The delalloc path does not
> trim after a short write, so in this case it also has to drop the
> folio and the delayed blocks that ->write_begin() set up past i_size.
> Otherwise that folio stays dirty past EOF and writeback never cleans
> it.
> 
> Fixes: 665575cff098 ("filemap: move prefaulting out of hot write path")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Hengyu Liang <hengyul@xxxxxxxxxx>

Hi Hengyu, thank you for the fix patch!

As Jan mentioned, this issue actually also exists on the iomap path,
although in my view the current approach can fix the problem on the
buffer_head path.

IIUC, 665575cff098 only increased the probability of the issue
occurring. Before it, copied could also end up being 0 in some
concurrent scenarios, because in block_write_end(), if the folio is not
uptodate, copied will be set to 0 as long as it is less than len.
Right?

> ---
>  fs/ext4/inode.c | 42 +++++++++++++++++++++++++++++++++++-------
>  1 file changed, 35 insertions(+), 7 deletions(-)
> 
> diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
> index 26f0f9714f03..36e75c89b8c8 100644
> --- a/fs/ext4/inode.c
> +++ b/fs/ext4/inode.c
> @@ -1452,15 +1452,18 @@ static int ext4_write_end(const struct kiocb *iocb,
>  	 * it's important to update i_size while still holding folio lock:
>  	 * page writeout could otherwise come in and zero beyond i_size.
>  	 *
> +	 * If nothing was copied (e.g. the source buffer was unreadable), do
> +	 * not move i_size up to pos.
> +	 *
>  	 * If FS_IOC_ENABLE_VERITY is running on this inode, then Merkle tree
>  	 * blocks are being written past EOF, so skip the i_size update.
>  	 */
> -	if (!verity)
> +	if (copied && !verity)
>  		i_size_changed = ext4_update_inode_size(inode, pos + copied);
>  	folio_unlock(folio);
>  	folio_put(folio);
>  
> -	if (old_size < pos && !verity)
> +	if (copied && old_size < pos && !verity)
>  		pagecache_isize_extended(inode, old_size, pos);
>  
>  	/*
> @@ -1571,13 +1574,13 @@ static int ext4_journalled_write_end(const struct kiocb *iocb,
>  		if (!partial)
>  			folio_mark_uptodate(folio);
>  	}
> -	if (!verity)
> +	if (copied && !verity)
>  		size_changed = ext4_update_inode_size(inode, pos + copied);
>  	EXT4_I(inode)->i_datasync_tid = handle->h_transaction->t_tid;
>  	folio_unlock(folio);
>  	folio_put(folio);
>  
> -	if (old_size < pos && !verity)
> +	if (copied && old_size < pos && !verity)
>  		pagecache_isize_extended(inode, old_size, pos);
>  
>  	if (size_changed) {
> @@ -3210,6 +3213,24 @@ static int ext4_da_should_update_i_disksize(struct folio *folio,
>  	return 1;
>  }
>  
> +/*
> + * Delalloc counterpart of ext4_truncate_failed_write(): ->write_begin() only
> + * reserved delayed blocks, so drop the page cache and the delayed extents
> + * beyond i_size and leave the blocks that are allocated there alone.
> + */
> +static void ext4_da_truncate_failed_write(struct inode *inode)
> +{
> +	struct address_space *mapping = inode->i_mapping;
> +	ext4_lblk_t lblk = EXT4_B_TO_LBLK(inode, inode->i_size);
> +
> +	filemap_invalidate_lock(mapping);
> +	truncate_inode_pages(mapping, inode->i_size);
> +	down_write(&EXT4_I(inode)->i_data_sem);
> +	ext4_es_remove_extent(inode, lblk, EXT_MAX_BLOCKS - lblk);
> +	up_write(&EXT4_I(inode)->i_data_sem);
> +	filemap_invalidate_unlock(mapping);
> +}
> +
>  static int ext4_da_do_write_end(struct address_space *mapping,
>  			loff_t pos, unsigned len, unsigned copied,
>  			struct folio *folio)
> @@ -3247,12 +3268,12 @@ static int ext4_da_do_write_end(struct address_space *mapping,
>  	 * checked, we need to update i_disksize here as certain
>  	 * ext4_writepages() paths not allocating blocks and update i_disksize.
>  	 */
> -	if (new_i_size > inode->i_size) {
> +	if (copied && new_i_size > inode->i_size) {
>  		unsigned long end;
>  
>  		i_size_write(inode, new_i_size);
>  		end = offset_in_folio(folio, new_i_size - 1);
> -		if (copied && ext4_da_should_update_i_disksize(folio, end)) {
> +		if (ext4_da_should_update_i_disksize(folio, end)) {
>  			ext4_update_i_disksize(inode, new_i_size);
>  			disksize_changed = true;
>  		}
> @@ -3261,9 +3282,16 @@ static int ext4_da_do_write_end(struct address_space *mapping,
>  	folio_unlock(folio);
>  	folio_put(folio);
>  
> -	if (pos > old_size)
> +	if (copied && pos > old_size)
>  		pagecache_isize_extended(inode, old_size, pos);
>  
> +	/*
> +	 * A write past i_size that copied nothing leaves i_size alone, so
> +	 * trim what ->write_begin() set up beyond it.
> +	 */
> +	if (!copied && pos > old_size)

I think it should be if (pos + len > old_size) here?

Thanks,
Yi.

> +		ext4_da_truncate_failed_write(inode);
> +
>  	if (!disksize_changed)
>  		return copied;
>