From: Zheng Liu Subject: Re: [RFC][PATCH 3/3] ext4: add dio overwrite nolock Date: Wed, 2 May 2012 16:16:26 +0800 Message-ID: <20120502081626.GB11639@gmail.com> References: <1335584346-8070-1-git-send-email-wenqing.lz@taobao.com> <1335584346-8070-4-git-send-email-wenqing.lz@taobao.com> <4FA0DB56.5000803@tao.ma> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: linux-ext4@vger.kernel.org To: Tao Ma Return-path: Received: from mail-yw0-f46.google.com ([209.85.213.46]:43540 "EHLO mail-yw0-f46.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752482Ab2EBIJf (ORCPT ); Wed, 2 May 2012 04:09:35 -0400 Received: by yhmm54 with SMTP id m54so376847yhm.19 for ; Wed, 02 May 2012 01:09:34 -0700 (PDT) Content-Disposition: inline In-Reply-To: <4FA0DB56.5000803@tao.ma> Sender: linux-ext4-owner@vger.kernel.org List-ID: On Wed, May 02, 2012 at 02:59:34PM +0800, Tao Ma wrote: > On 04/28/2012 11:39 AM, Zheng Liu wrote: > > From: Zheng Liu > > > > 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 > > --- > > 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. Yes, you are right. > > + 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? OK. I will add some comments to describe it in here. > > + 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? Yes, the metadata is possible to be changed after we unlocked i_mutex before acquire i_data_sem. So I will swap the locking order. > > + } > > + } > > + > > + 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? As I said above, if we swap the locking order, I think that it shouldn't happen. Certainly, I will set 'iocb->private = NULL' to fix it to ensure that when it does happen, we can make filesystem do right things. > > + } > > + > > + 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? No, we don't need to re-lock i_mutex because dio never falls through to buffered IO when it is an overwrite. We do a lookup using ext4_map_blocks to ensure that it never occurs before we actually issue a dio. I will add a BUG_ON to guarantee that it couldn't happen. Regards, Zheng > > 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)); >