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Message-ID: <4FA0DB56.5000803@tao.ma>
Date: Wed, 02 May 2012 14:59:34 +0800
From: Tao Ma <tm@....ma>
To: Zheng Liu <gnehzuil.liu@...il.com>
CC: linux-ext4@...r.kernel.org
Subject: Re: [RFC][PATCH 3/3] ext4: add dio overwrite nolock
On 04/28/2012 11:39 AM, Zheng Liu wrote:
> From: Zheng Liu <wenqing.lz@...bao.com>
>
> Aligned and overwrite direct IO can be parallelized. In ext4_file_dio_write,
> we first check whether these conditions are satisfied or not. If so, we unlock
> the i_mutex and acquire i_data_sem directly. Meanwhile iocb->private is set to
> indicate that this is a overwrite dio, and it will be processed in
> ext4_ext_direct_IO.
>
> Signed-off-by: Zheng Liu <wenqing.lz@...bao.com>
> ---
> fs/ext4/file.c | 140 ++++++++++++++++++++++++++++++++++++++++++++++++++++++-
> 1 files changed, 137 insertions(+), 3 deletions(-)
>
> diff --git a/fs/ext4/file.c b/fs/ext4/file.c
> index e5d6be3..8a5f713 100644
> --- a/fs/ext4/file.c
> +++ b/fs/ext4/file.c
> @@ -100,9 +100,21 @@ static ssize_t
> ext4_file_dio_write(struct kiocb *iocb, const struct iovec *iov,
> unsigned long nr_segs, loff_t pos)
> {
> - struct inode *inode = iocb->ki_filp->f_path.dentry->d_inode;
> - int unaligned_aio = 0;
> + struct file *file = iocb->ki_filp;
> + struct address_space * mapping = file->f_mapping;
> + struct inode *inode = file->f_path.dentry->d_inode;
> + struct blk_plug plug;
> ssize_t ret;
> + ssize_t written, written_buffered;
> + size_t length = iov_length(iov, nr_segs);
> + size_t ocount; /* original count */
> + size_t count; /* after file limit checks */
> + int unaligned_aio = 0;
> + int overwrite = 0;
> + loff_t *ppos = &iocb->ki_pos;
> + loff_t endbyte;
> +
> + BUG_ON(iocb->ki_pos != pos);
>
> if (!is_sync_kiocb(iocb))
> unaligned_aio = ext4_unaligned_aio(inode, iov, nr_segs, pos);
> @@ -121,7 +133,129 @@ ext4_file_dio_write(struct kiocb *iocb, const struct iovec *iov,
> ext4_aiodio_wait(inode);
> }
>
> - ret = generic_file_aio_write(iocb, iov, nr_segs, pos);
> + mutex_lock(&inode->i_mutex);
> + blk_start_plug(&plug);
> +
> + ocount = 0;
> + ret = generic_segment_checks(iov, &nr_segs, &ocount, VERIFY_READ);
> + if (ret)
> + goto unlock_out;
> +
> + count = ocount;
> + pos = *ppos;
> +
> + vfs_check_frozen(inode->i_sb, SB_FREEZE_WRITE);
> +
> + /* We can write back this queue in page reclaim */
> + current->backing_dev_info = mapping->backing_dev_info;
> + written = 0;
> +
> + ret = generic_write_checks(file, &pos, &count, S_ISBLK(inode->i_mode));
> + if (ret)
> + goto out;
> +
> + if (count == 0)
> + goto out;
> +
> + ret = file_remove_suid(file);
> + if (ret)
> + goto out;
> +
> + file_update_time(file);
> +
> + iocb->private = NULL;
> +
> + if (!unaligned_aio && !file->f_mapping->nrpages &&
> + pos + length < i_size_read(inode) &&
should be pos + length <= ?
And inode->i_size should be ok since now we have i_mutex held.
> + ext4_should_dioread_nolock(inode)) {
> + struct ext4_map_blocks map;
> + unsigned int blkbits = inode->i_blkbits;
> + int err;
> + int len;
> +
> + map.m_lblk = pos >> blkbits;
> + map.m_len = (EXT4_BLOCK_ALIGN(pos + length, blkbits) >> blkbits)
> + - map.m_lblk;
> + len = map.m_len;
> +
> + err = ext4_map_blocks(NULL, inode, &map, 0);
> + if (err == len && (!map.m_flags ||
> + map.m_flags & EXT4_MAP_MAPPED)) {
could you please add some comments about how and why map.m_flags are
checked this way?
> + overwrite = 1;
> + iocb->private = &overwrite;
> + mutex_unlock(&inode->i_mutex);
> + down_read(&EXT4_I(inode)->i_data_sem);
Is there any possibility that the metadata is changed after we dropped
the i_mutex before the down_read?
> + }
> + }
> +
> + if (file->f_mapping->nrpages && overwrite) {
> + overwrite = 0;
> + up_read(&EXT4_I(inode)->i_data_sem);
> + mutex_lock(&inode->i_mutex);
I am not sure whether it could happen. But if it does happen, should we
also change the value in iocb->private?
> + }
> +
> + written = generic_file_direct_write(iocb, iov, &nr_segs, pos,
> + ppos, count, ocount);
> + if (written < 0 || written == count)
> + goto out;
> + /*
> + * direct-io write to a hole: fall through to buffered I/O
> + * for completing the rest of the request.
> + */
> + pos += written;
> + count -= written;
> + written_buffered = generic_file_buffered_write(iocb, iov,
> + nr_segs, pos, ppos, count,
> + written);
If we fall back here, should we re-lock the i_mutex since the buffer
write isn't guaranteed?
Thanks
Tao
> + /*
> + * If generic_file_buffered_write() retuned a synchronous error
> + * then we want to return the number of bytes which were
> + * direct-written, or the error code if that was zero. Note
> + * that this differs from normal direct-io semantics, which
> + * will return -EFOO even if some bytes were written.
> + */
> + if (written_buffered < 0) {
> + ret = written_buffered;
> + goto out;
> + }
> +
> + /*
> + * We need to ensure that the page cache pages are written to
> + * disk and invalidated to preserve the expected O_DIRECT
> + * semantics.
> + */
> + endbyte = pos + written_buffered - written - 1;
> + ret = filemap_write_and_wait_range(file->f_mapping, pos, endbyte);
> + if (ret == 0) {
> + written = written_buffered;
> + invalidate_mapping_pages(mapping,
> + pos >> PAGE_CACHE_SHIFT,
> + endbyte >> PAGE_CACHE_SHIFT);
> + } else {
> + /*
> + * We don't know how much we wrote, so just return
> + * the number of bytes which were direct-written
> + */
> + }
> +
> +out:
> + current->backing_dev_info = NULL;
> + ret = written ? written : ret;
> +
> +unlock_out:
> + if (overwrite)
> + up_read(&EXT4_I(inode)->i_data_sem);
> + else
> + mutex_unlock(&inode->i_mutex);
> +
> + if (ret > 0 || ret == -EIOCBQUEUED) {
> + ssize_t err;
> +
> + err = generic_write_sync(file, pos, ret);
> + if (err < 0 && ret > 0)
> + ret = err;
> + }
> + blk_finish_plug(&plug);
>
> if (unaligned_aio)
> mutex_unlock(ext4_aio_mutex(inode));
--
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