On Mon 12-08-19 22:53:26, Matthew Bobrowski wrote:
> This patch introduces a new direct IO write code path implementation
> that makes use of the iomap infrastructure.
>
> All direct IO write operations are now passed from the ->write_iter() callback
> to the new function ext4_dio_write_iter(). This function is responsible for
> calling into iomap infrastructure via iomap_dio_rw(). Snippets of the direct
> IO code from within ext4_file_write_iter(), such as checking whether the IO
> request is unaligned asynchronous IO, or whether it will ber overwriting
> allocated and initialized blocks has been moved out and into
> ext4_dio_write_iter().
>
> The block mapping flags that are passed to ext4_map_blocks() from within
> ext4_dio_get_block() and friends have effectively been taken out and
> introduced within the ext4_iomap_begin(). If ext4_map_blocks() happens to have
> instantiated blocks beyond the i_size, then we attempt to place the inode onto
> the orphan list. Despite being able to perform i_size extension checking
> earlier on in the direct IO code path, it makes most sense to perform this bit
> post successful block allocation.
>
> The ->end_io() callback ext4_dio_write_end_io() is responsible for removing
> the inode from the orphan list and determining if we should truncate a failed
> write in the case of an error. We also convert a range of unwritten extents to
> written if IOMAP_DIO_UNWRITTEN is set and perform the necessary
> i_size/i_disksize extension if the iocb->ki_pos + dio->size > i_size_read(inode).
>
> In the instance of a short write, we fallback to buffered IO and complete
> whatever is left the 'iter'. Any blocks that may have been allocated in
> preparation for direct IO will be reused by buffered IO, so there's no issue
> with leaving allocated blocks beyond EOF.
>
> Signed-off-by: Matthew Bobrowski <mbobrowski@xxxxxxxxxxxxxx>
> ---
> fs/ext4/file.c | 227 ++++++++++++++++++++++++++++++++++++++++----------------
> fs/ext4/inode.c | 42 +++++++++--
> 2 files changed, 199 insertions(+), 70 deletions(-)
Overall this is very nice. Some smaller comments below.
> @@ -235,6 +244,34 @@ static ssize_t ext4_write_checks(struct kiocb *iocb, struct iov_iter *from)
> return iov_iter_count(from);
> }
>
> +static ssize_t ext4_buffered_write_iter(struct kiocb *iocb,
> + struct iov_iter *from)
> +{
> + ssize_t ret;
> + struct inode *inode = file_inode(iocb->ki_filp);
> +
> + if (!inode_trylock(inode)) {
> + if (iocb->ki_flags & IOCB_NOWAIT)
> + return -EOPNOTSUPP;
> + inode_lock(inode);
> + }
Currently there's no support for IOCB_NOWAIT for buffered IO so you can
replace this with "inode_lock(inode)".
> @@ -284,6 +321,128 @@ static int ext4_handle_inode_extension(struct inode *inode, loff_t size,
> return ret;
> }
>
I'd mention here that for cases where inode size is extended,
ext4_dio_write_iter() waits for DIO to complete and thus we are protected
by inode_lock in that case.
> +static int ext4_dio_write_end_io(struct kiocb *iocb, ssize_t size,
> + ssize_t error, unsigned int flags)
> +{
> + int ret = 0;
> + handle_t *handle;
> + loff_t offset = iocb->ki_pos;
> + struct inode *inode = file_inode(iocb->ki_filp);
> +
> + if (error) {
> + if (offset + size > i_size_read(inode))
> + ext4_truncate_failed_write(inode);
> +
> + /*
> + * The inode may have been placed onto the orphan list
> + * as a result of an extension. However, an error may
> + * have been encountered prior to being able to
> + * complete the write operation. Perform any necessary
> + * clean up in this case.
> + */
> + if (!list_empty(&EXT4_I(inode)->i_orphan)) {
> + handle = ext4_journal_start(inode, EXT4_HT_INODE, 2);
> + if (IS_ERR(handle)) {
> + if (inode->i_nlink)
> + ext4_orphan_del(NULL, inode);
> + return PTR_ERR(handle);
> + }
> +
> + if (inode->i_nlink)
> + ext4_orphan_del(handle, inode);
> + ext4_journal_stop(handle);
> + }
> + return error;
> + }
> +
> + if (flags & IOMAP_DIO_UNWRITTEN) {
> + ret = ext4_convert_unwritten_extents(NULL, inode, offset, size);
> + if (ret)
> + return ret;
> + }
> +
> + if (offset + size > i_size_read(inode)) {
> + ret = ext4_handle_inode_extension(inode, offset + size, 0);
> + if (ret)
> + return ret;
> + }
> + return ret;
> +}
> +
> +static ssize_t ext4_dio_write_iter(struct kiocb *iocb, struct iov_iter *from)
> +{
> + ssize_t ret;
> + loff_t offset = iocb->ki_pos;
> + size_t count = iov_iter_count(from);
> + struct inode *inode = file_inode(iocb->ki_filp);
> + bool extend = false, overwrite = false, unaligned_aio = false;
> +
> + if (!inode_trylock(inode)) {
> + if (iocb->ki_flags & IOCB_NOWAIT)
> + return -EAGAIN;
> + inode_lock(inode);
> + }
> +
> + if (!ext4_dio_checks(inode)) {
> + inode_unlock(inode);
> + /*
> + * Fallback to buffered IO if the operation on the
> + * inode is not supported by direct IO.
> + */
> + return ext4_buffered_write_iter(iocb, from);
> + }
> +
> + ret = ext4_write_checks(iocb, from);
> + if (ret <= 0)
> + goto out;
> +
> + /*
> + * Unaligned direct AIO must be serialized among each other as
> + * the zeroing of partial blocks of two competing unaligned
> + * AIOs can result in data corruption.
> + */
> + if (ext4_test_inode_flag(inode, EXT4_INODE_EXTENTS) &&
> + !is_sync_kiocb(iocb) && ext4_unaligned_aio(inode, from, offset)) {
> + unaligned_aio = true;
> + inode_dio_wait(inode);
> + }
> +
> + /*
> + * Determine whether the IO operation will overwrite allocated
> + * and initialized blocks. If so, check to see whether it is
> + * possible to take the dioread_nolock path.
> + */
> + if (!unaligned_aio && ext4_overwrite_io(inode, offset, count) &&
> + ext4_should_dioread_nolock(inode)) {
> + overwrite = true;
> + downgrade_write(&inode->i_rwsem);
> + }
> +
> + if (offset + count > i_size_read(inode) ||
> + offset + count > EXT4_I(inode)->i_disksize) {
> + ext4_update_i_disksize(inode, inode->i_size);
> + extend = true;
> + }
> +
> + ret = iomap_dio_rw(iocb, from, &ext4_iomap_ops, ext4_dio_write_end_io);
> +
> + /*
> + * Unaligned direct AIO must be the only IO in flight or else
> + * any overlapping aligned IO after unaligned IO might result
> + * in data corruption.
> + */
Here I'd expand the comment to explain that we wait in case inode is
extended so that inode extension in ext4_dio_write_end_io() is properly
covered by inode_lock.
> + if (ret == -EIOCBQUEUED && (unaligned_aio || extend))
> + inode_dio_wait(inode);
> +
> + if (ret >= 0 && iov_iter_count(from)) {
> + overwrite ? inode_unlock_shared(inode) : inode_unlock(inode);
> + return ext4_buffered_write_iter(iocb, from);
> + }
> +out:
> + overwrite ? inode_unlock_shared(inode) : inode_unlock(inode);
> + return ret;
> +}
> +
> #ifdef CONFIG_FS_DAX
> static ssize_t
> ext4_dax_write_iter(struct kiocb *iocb, struct iov_iter *from)
...
> @@ -3581,10 +3611,10 @@ static int ext4_iomap_begin(struct inode *inode, loff_t offset, loff_t length,
> iomap->type = delalloc ? IOMAP_DELALLOC : IOMAP_HOLE;
> iomap->addr = IOMAP_NULL_ADDR;
> } else {
> - if (map.m_flags & EXT4_MAP_MAPPED) {
> - iomap->type = IOMAP_MAPPED;
> - } else if (map.m_flags & EXT4_MAP_UNWRITTEN) {
> + if (map.m_flags & EXT4_MAP_UNWRITTEN) {
> iomap->type = IOMAP_UNWRITTEN;
> + } else if (map.m_flags & EXT4_MAP_MAPPED) {
> + iomap->type = IOMAP_MAPPED;
> } else {
> WARN_ON_ONCE(1);
> return -EIO;
Possibly this hunk should go into a separate patch (since this is not
directly related with iomap conversion) with a changelog / comment
explaining why we need to check EXT4_MAP_UNWRITTEN first.
Honza
--
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR
