Re: [PATCH v3 5/7] ext4: Add multi-fsblock atomic write support with bigalloc

["Darrick J. Wong" <djwong@kernel.org>]

On Fri, May 09, 2025 at 02:20:35AM +0530, Ritesh Harjani (IBM) wrote:
> EXT4 supports bigalloc feature which allows the FS to work in size of
> clusters (group of blocks) rather than individual blocks. This patch
> adds atomic write support for bigalloc so that systems with bs = ps can
> also create FS using -
>     mkfs.ext4 -F -O bigalloc -b 4096 -C 16384 <dev>
> 
> With bigalloc ext4 can support multi-fsblock atomic writes. We will have to
> adjust ext4's atomic write unit max value to cluster size. This can then support
> atomic write of size anywhere between [blocksize, clustersize]. This
> patch adds the required changes to enable multi-fsblock atomic write
> support using bigalloc in the next patch.
> 
> In this patch for block allocation:
> we first query the underlying region of the requested range by calling
> ext4_map_blocks() call. Here are the various cases which we then handle
> depending upon the underlying mapping type:
> 1. If the underlying region for the entire requested range is a mapped extent,
>    then we don't call ext4_map_blocks() to allocate anything. We don't need to
>    even start the jbd2 txn in this case.
> 2. For an append write case, we create a mapped extent.
> 3. If the underlying region is entirely a hole, then we create an unwritten
>    extent for the requested range.
> 4. If the underlying region is a large unwritten extent, then we split the
>    extent into 2 unwritten extent of required size.
> 5. If the underlying region has any type of mixed mapping, then we call
>    ext4_map_blocks() in a loop to zero out the unwritten and the hole regions
>    within the requested range. This then provide a single mapped extent type
>    mapping for the requested range.
> 
> Note: We invoke ext4_map_blocks() in a loop with the EXT4_GET_BLOCKS_ZERO
> flag only when the underlying extent mapping of the requested range is
> not entirely a hole, an unwritten extent, or a fully mapped extent. That
> is, if the underlying region contains a mix of hole(s), unwritten
> extent(s), and mapped extent(s), we use this loop to ensure that all the
> short mappings are zeroed out. This guarantees that the entire requested
> range becomes a single, uniformly mapped extent. It is ok to do so
> because we know this is being done on a bigalloc enabled filesystem
> where the block bitmap represents the entire cluster unit.
> 
> Note having a single contiguous underlying region of type mapped,
> unwrittn or hole is not a problem. But the reason to avoid writing on
> top of mixed mapping region is because, atomic writes requires all or
> nothing should get written for the userspace pwritev2 request. So if at
> any point in time during the write if a crash or a sudden poweroff
> occurs, the region undergoing atomic write should read either complete
> old data or complete new data. But it should never have a mix of both
> old and new data.
> So, we first convert any mixed mapping region to a single contiguous
> mapped extent before any data gets written to it. This is because
> normally FS will only convert unwritten extents to written at the end of
> the write in ->end_io() call. And if we allow the writes over a mixed
> mapping and if a sudden power off happens in between, we will end up
> reading mix of new data (over mapped extents) and old data (over
> unwritten extents), because unwritten to written conversion never went
> through.
> So to avoid this and to avoid writes getting torned due to mixed
> mapping, we first allocate a single contiguous block mapping and then
> do the write.
> 
> Co-developed-by: Ojaswin Mujoo <ojaswin@linux.ibm.com>
> Signed-off-by: Ojaswin Mujoo <ojaswin@linux.ibm.com>
> Signed-off-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>

Looks fine (I don't like the pre-zeroing but options are limited on
ext4) except for one thing...

> diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
> index 8b86b1a29bdc..2642e1ef128f 100644
> --- a/fs/ext4/inode.c
> +++ b/fs/ext4/inode.c
> @@ -3412,6 +3412,136 @@ static void ext4_set_iomap(struct inode *inode, struct iomap *iomap,
>  	}
>  }
>  
> +static int ext4_map_blocks_atomic_write_slow(handle_t *handle,
> +			struct inode *inode, struct ext4_map_blocks *map)
> +{
> +	ext4_lblk_t m_lblk = map->m_lblk;
> +	unsigned int m_len = map->m_len;
> +	unsigned int mapped_len = 0, m_flags = 0;
> +	ext4_fsblk_t next_pblk;
> +	bool check_next_pblk = false;
> +	int ret = 0;
> +
> +	WARN_ON_ONCE(!ext4_has_feature_bigalloc(inode->i_sb));
> +
> +	/*
> +	 * This is a slow path in case of mixed mapping. We use
> +	 * EXT4_GET_BLOCKS_CREATE_ZERO flag here to make sure we get a single
> +	 * contiguous mapped mapping. This will ensure any unwritten or hole
> +	 * regions within the requested range is zeroed out and we return
> +	 * a single contiguous mapped extent.
> +	 */
> +	m_flags = EXT4_GET_BLOCKS_CREATE_ZERO;
> +
> +	do {
> +		ret = ext4_map_blocks(handle, inode, map, m_flags);
> +		if (ret < 0 && ret != -ENOSPC)
> +			goto out_err;
> +		/*
> +		 * This should never happen, but let's return an error code to
> +		 * avoid an infinite loop in here.
> +		 */
> +		if (ret == 0) {
> +			ret = -EFSCORRUPTED;
> +			ext4_warning_inode(inode,
> +				"ext4_map_blocks() couldn't allocate blocks m_flags: 0x%x, ret:%d",
> +				m_flags, ret);
> +			goto out_err;
> +		}
> +		/*
> +		 * With bigalloc we should never get ENOSPC nor discontiguous
> +		 * physical extents.
> +		 */
> +		if ((check_next_pblk && next_pblk != map->m_pblk) ||
> +				ret == -ENOSPC) {
> +			ext4_warning_inode(inode,
> +				"Non-contiguous allocation detected: expected %llu, got %llu, "
> +				"or ext4_map_blocks() returned out of space ret: %d",
> +				next_pblk, map->m_pblk, ret);
> +			ret = -ENOSPC;
> +			goto out_err;

If you get physically discontiguous mappings within a cluster, the
extent tree is corrupt.

--D

> +		}
> +		next_pblk = map->m_pblk + map->m_len;
> +		check_next_pblk = true;
> +
> +		mapped_len += map->m_len;
> +		map->m_lblk += map->m_len;
> +		map->m_len = m_len - mapped_len;
> +	} while (mapped_len < m_len);
> +
> +	/*
> +	 * We might have done some work in above loop, so we need to query the
> +	 * start of the physical extent, based on the origin m_lblk and m_len.
> +	 * Let's also ensure we were able to allocate the required range for
> +	 * mixed mapping case.
> +	 */
> +	map->m_lblk = m_lblk;
> +	map->m_len = m_len;
> +	map->m_flags = 0;
> +
> +	ret = ext4_map_blocks(handle, inode, map,
> +			      EXT4_GET_BLOCKS_QUERY_LAST_IN_LEAF);
> +	if (ret != m_len) {
> +		ext4_warning_inode(inode,
> +			"allocation failed for atomic write request m_lblk:%u, m_len:%u, ret:%d\n",
> +			m_lblk, m_len, ret);
> +		ret = -EINVAL;
> +	}
> +	return ret;
> +
> +out_err:
> +	/* reset map before returning an error */
> +	map->m_lblk = m_lblk;
> +	map->m_len = m_len;
> +	map->m_flags = 0;
> +	return ret;
> +}
> +
> +/*
> + * ext4_map_blocks_atomic: Helper routine to ensure the entire requested
> + * range in @map [lblk, lblk + len) is one single contiguous extent with no
> + * mixed mappings.
> + *
> + * We first use m_flags passed to us by our caller (ext4_iomap_alloc()).
> + * We only call EXT4_GET_BLOCKS_ZERO in the slow path, when the underlying
> + * physical extent for the requested range does not have a single contiguous
> + * mapping type i.e. (Hole, Mapped, or Unwritten) throughout.
> + * In that case we will loop over the requested range to allocate and zero out
> + * the unwritten / holes in between, to get a single mapped extent from
> + * [m_lblk, m_lblk +  m_len). Note that this is only possible because we know
> + * this can be called only with bigalloc enabled filesystem where the underlying
> + * cluster is already allocated. This avoids allocating discontiguous extents
> + * in the slow path due to multiple calls to ext4_map_blocks().
> + * The slow path is mostly non-performance critical path, so it should be ok to
> + * loop using ext4_map_blocks() with appropriate flags to allocate & zero the
> + * underlying short holes/unwritten extents within the requested range.
> + */
> +static int ext4_map_blocks_atomic_write(handle_t *handle, struct inode *inode,
> +				struct ext4_map_blocks *map, int m_flags)
> +{
> +	ext4_lblk_t m_lblk = map->m_lblk;
> +	unsigned int m_len = map->m_len;
> +	int ret = 0;
> +
> +	WARN_ON_ONCE(m_len > 1 && !ext4_has_feature_bigalloc(inode->i_sb));
> +
> +	ret = ext4_map_blocks(handle, inode, map, m_flags);
> +	if (ret < 0 || ret == m_len)
> +		goto out;
> +	/*
> +	 * This is a mixed mapping case where we were not able to allocate
> +	 * a single contiguous extent. In that case let's reset requested
> +	 * mapping and call the slow path.
> +	 */
> +	map->m_lblk = m_lblk;
> +	map->m_len = m_len;
> +	map->m_flags = 0;
> +
> +	return ext4_map_blocks_atomic_write_slow(handle, inode, map);
> +out:
> +	return ret;
> +}
> +
>  static int ext4_iomap_alloc(struct inode *inode, struct ext4_map_blocks *map,
>  			    unsigned int flags)
>  {
> @@ -3425,7 +3555,30 @@ static int ext4_iomap_alloc(struct inode *inode, struct ext4_map_blocks *map,
>  	 */
>  	if (map->m_len > DIO_MAX_BLOCKS)
>  		map->m_len = DIO_MAX_BLOCKS;
> -	dio_credits = ext4_chunk_trans_blocks(inode, map->m_len);
> +
> +	/*
> +	 * journal credits estimation for atomic writes. We call
> +	 * ext4_map_blocks(), to find if there could be a mixed mapping. If yes,
> +	 * then let's assume the no. of pextents required can be m_len i.e.
> +	 * every alternate block can be unwritten and hole.
> +	 */
> +	if (flags & IOMAP_ATOMIC) {
> +		unsigned int orig_mlen = map->m_len;
> +
> +		ret = ext4_map_blocks(NULL, inode, map, 0);
> +		if (ret < 0)
> +			return ret;
> +		if (map->m_len < orig_mlen) {
> +			map->m_len = orig_mlen;
> +			dio_credits = ext4_meta_trans_blocks(inode, orig_mlen,
> +							     map->m_len);
> +		} else {
> +			dio_credits = ext4_chunk_trans_blocks(inode,
> +							      map->m_len);
> +		}
> +	} else {
> +		dio_credits = ext4_chunk_trans_blocks(inode, map->m_len);
> +	}
>  
>  retry:
>  	/*
> @@ -3456,7 +3609,10 @@ static int ext4_iomap_alloc(struct inode *inode, struct ext4_map_blocks *map,
>  	else if (ext4_test_inode_flag(inode, EXT4_INODE_EXTENTS))
>  		m_flags = EXT4_GET_BLOCKS_IO_CREATE_EXT;
>  
> -	ret = ext4_map_blocks(handle, inode, map, m_flags);
> +	if (flags & IOMAP_ATOMIC)
> +		ret = ext4_map_blocks_atomic_write(handle, inode, map, m_flags);
> +	else
> +		ret = ext4_map_blocks(handle, inode, map, m_flags);
>  
>  	/*
>  	 * We cannot fill holes in indirect tree based inodes as that could
> @@ -3480,6 +3636,7 @@ static int ext4_iomap_begin(struct inode *inode, loff_t offset, loff_t length,
>  	int ret;
>  	struct ext4_map_blocks map;
>  	u8 blkbits = inode->i_blkbits;
> +	unsigned int orig_mlen;
>  
>  	if ((offset >> blkbits) > EXT4_MAX_LOGICAL_BLOCK)
>  		return -EINVAL;
> @@ -3493,6 +3650,7 @@ static int ext4_iomap_begin(struct inode *inode, loff_t offset, loff_t length,
>  	map.m_lblk = offset >> blkbits;
>  	map.m_len = min_t(loff_t, (offset + length - 1) >> blkbits,
>  			  EXT4_MAX_LOGICAL_BLOCK) - map.m_lblk + 1;
> +	orig_mlen = map.m_len;
>  
>  	if (flags & IOMAP_WRITE) {
>  		/*
> @@ -3503,8 +3661,16 @@ static int ext4_iomap_begin(struct inode *inode, loff_t offset, loff_t length,
>  		 */
>  		if (offset + length <= i_size_read(inode)) {
>  			ret = ext4_map_blocks(NULL, inode, &map, 0);
> -			if (ret > 0 && (map.m_flags & EXT4_MAP_MAPPED))
> -				goto out;
> +			/*
> +			 * For atomic writes the entire requested length should
> +			 * be mapped.
> +			 */
> +			if (map.m_flags & EXT4_MAP_MAPPED) {
> +				if ((!(flags & IOMAP_ATOMIC) && ret > 0) ||
> +				   (flags & IOMAP_ATOMIC && ret >= orig_mlen))
> +					goto out;
> +			}
> +			map.m_len = orig_mlen;
>  		}
>  		ret = ext4_iomap_alloc(inode, &map, flags);
>  	} else {
> @@ -3525,6 +3691,16 @@ static int ext4_iomap_begin(struct inode *inode, loff_t offset, loff_t length,
>  	 */
>  	map.m_len = fscrypt_limit_io_blocks(inode, map.m_lblk, map.m_len);
>  
> +	/*
> +	 * Before returning to iomap, let's ensure the allocated mapping
> +	 * covers the entire requested length for atomic writes.
> +	 */
> +	if (flags & IOMAP_ATOMIC) {
> +		if (map.m_len < (length >> blkbits)) {
> +			WARN_ON(1);
> +			return -EINVAL;
> +		}
> +	}
>  	ext4_set_iomap(inode, iomap, &map, offset, length, flags);
>  
>  	return 0;
> -- 
> 2.49.0
> 
>