[gnehzuil.liu@gmail.com: Re: [RFC][PATCH 3/3] ext4: add dio overwrite nolock]

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----- Forwarded message from Zheng Liu [off-list ref] -----

Date: Wed, 2 May 2012 23:49:35 +0800
From: Zheng Liu <redacted>
To: Eric Sandeen <redacted>
Cc: "linux-ext4@vger.kernel.org" <redacted>, Zheng Liu <redacted>
Subject: Re: [RFC][PATCH 3/3] ext4: add dio overwrite nolock

On Wednesday, May 2, 2012, Eric Sandeen wrote:

On 4/27/12 10:39 PM, Zheng Liu wrote:

quoted

From: Zheng Liu <wenqing.lz@taobao.com <javascript:;>>

Aligned and overwrite direct IO can be parallelized.  In

ext4_file_dio_write,

quoted

we first check whether these conditions are satisfied or not.  If so, we

unlock

quoted

the i_mutex and acquire i_data_sem directly.  Meanwhile iocb->private is

set to

quoted

indicate that this is a overwrite dio, and it will be processed in
ext4_ext_direct_IO.

This copies almost 100 lines of generic_file_aio_write() back into
ext4.  Do we really need to do this?  Copying core code into the
fs can be a maintenance nightmare...

I'll have to think more about the big picture and whether or not it's
possible, but my first reaction is to find a way to leverage or modify
existing IO code rather than pasting it all into ext4 with changes...

The key issue is file_update_time.  In this function, it starts a new
journal.  It will cause a deadlock because we usually start a new journal
firstly, and then try to lock i_data_sem.  But in overwrite this order is
reversed.  If we can avoid this deadlock, we won't need to copy core code
into ext4.

Regards,
Zheng

-Eric

quoted

Signed-off-by: Zheng Liu <redacted>
---
 fs/ext4/file.c |  140

++++++++++++++++++++++++++++++++++++++++++++++++++++++-

quoted

 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);

quoted

@@ -121,7 +133,129 @@ ext4_file_dio_write(struct kiocb *iocb, const

struct iovec *iov,

quoted

              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));

quoted

+     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) &&
+         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)

quoted

+                     - 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)) {
+

----- End forwarded message -----