Re: [RFC 1/1] ext4: Optimize ext4 DAX overwrites

[Ritesh Harjani]

On 8/20/20 6:23 PM, Jan Kara wrote:
> On Thu 20-08-20 17:06:28, Ritesh Harjani wrote:
> > Currently in case of DAX, we are starting a transaction
> > everytime for IOMAP_WRITE case. This can be optimized
> > away in case of an overwrite (where the blocks were already
> > allocated). This could give a significant performance boost
> > for multi-threaded random writes.
> >
> > Reported-by: Dan Williams <dan.j.williams@xxxxxxxxx>
> > Signed-off-by: Ritesh Harjani <riteshh@xxxxxxxxxxxxx>
>
> Thanks for returning to this and I'm glad to see how much this helped :)
> BTW, I'd suspect there could be also significant contention and cache line
> bouncing on j_state_lock and transaction's atomic counters...

ok, will try and profile to see if this happens.


> > ---
> >   fs/ext4/ext4.h  | 1 +
> >   fs/ext4/file.c  | 2 +-
> >   fs/ext4/inode.c | 8 +++++++-
> >   3 files changed, 9 insertions(+), 2 deletions(-)
> >
> > diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
> > index 42f5060f3cdf..9a2138afc751 100644
> > --- a/fs/ext4/ext4.h
> > +++ b/fs/ext4/ext4.h
> > @@ -3232,6 +3232,7 @@ extern const struct dentry_operations ext4_dentry_ops;
> >   extern const struct inode_operations ext4_file_inode_operations;
> >   extern const struct file_operations ext4_file_operations;
> >   extern loff_t ext4_llseek(struct file *file, loff_t offset, int origin);
> > +extern bool ext4_overwrite_io(struct inode *inode, loff_t pos, loff_t len);
> >   
> >   /* inline.c */
> >   extern int ext4_get_max_inline_size(struct inode *inode);
> > diff --git a/fs/ext4/file.c b/fs/ext4/file.c
> > index 2a01e31a032c..51cd92ac1758 100644
> > --- a/fs/ext4/file.c
> > +++ b/fs/ext4/file.c
> > @@ -188,7 +188,7 @@ ext4_extending_io(struct inode *inode, loff_t offset, size_t len)
> >   }
> >   
> >   /* Is IO overwriting allocated and initialized blocks? */
> > -static bool ext4_overwrite_io(struct inode *inode, loff_t pos, loff_t len)
> > +bool ext4_overwrite_io(struct inode *inode, loff_t pos, loff_t len)
> >   {
> >   	struct ext4_map_blocks map;
> >   	unsigned int blkbits = inode->i_blkbits;
> > diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
> > index 10dd470876b3..f0ac0ee9e991 100644
> > --- a/fs/ext4/inode.c
> > +++ b/fs/ext4/inode.c
> > @@ -3423,6 +3423,7 @@ 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;
> > +	bool overwrite = false;
> >   
> >   	if ((offset >> blkbits) > EXT4_MAX_LOGICAL_BLOCK)
> >   		return -EINVAL;
> > @@ -3430,6 +3431,9 @@ static int ext4_iomap_begin(struct inode *inode, loff_t offset, loff_t length,
> >   	if (WARN_ON_ONCE(ext4_has_inline_data(inode)))
> >   		return -ERANGE;
> >   
> > +	if (IS_DAX(inode) && (flags & IOMAP_WRITE) &&
> > +	    ext4_overwrite_io(inode, offset, length))
> > +		overwrite = true;
>
> So the patch looks correct but using ext4_overwrite_io() seems a bit
> foolish since under the hood it does ext4_map_blocks() only to be able to
> decide whether to call ext4_map_blocks() once again with exactly the same
> arguments :). So I'd rather slightly refactor the code in
> ext4_iomap_begin() to avoid this double calling of ext4_map_blocks() for
> the fast path.

Yes, agreed. Looking at the numbers I was excited to post out the RFC
for discussion. Will make above changes and post. :)


With DIO, we need to detect overwrite case early in
ext4_dio_write_iter() to determine whether we need shared or excl.
locks - so probably for DIO case we still need overwrite check in
ext4_dio_write_iter()

Thanks for review!!
-ritesh


> 								Honza
>
> >   	/*
> >   	 * Calculate the first and last logical blocks respectively.
> >   	 */
> > @@ -3437,13 +3441,15 @@ static int ext4_iomap_begin(struct inode *inode, loff_t offset, loff_t length,
> >   	map.m_len = min_t(loff_t, (offset + length - 1) >> blkbits,
> >   			  EXT4_MAX_LOGICAL_BLOCK) - map.m_lblk + 1;
> >   
> > -	if (flags & IOMAP_WRITE)
> > +	if ((flags & IOMAP_WRITE) && !overwrite)
> >   		ret = ext4_iomap_alloc(inode, &map, flags);
> >   	else
> >   		ret = ext4_map_blocks(NULL, inode, &map, 0);
> >   
> >   	if (ret < 0)
> >   		return ret;
> > +	if (IS_DAX(inode) && overwrite)
> > +		WARN_ON(!(map.m_flags & EXT4_MAP_MAPPED));
> >   
> >   	ext4_set_iomap(inode, iomap, &map, offset, length);
> >   
> > --
> > 2.25.4