Re: [PATCH 16/19] ext4: Support for synchronous DAX faults

[Jan Kara <jack@suse.cz>]

On Fri 13-10-17 14:58:54, Ross Zwisler wrote:
> On Wed, Oct 11, 2017 at 10:06:00PM +0200, Jan Kara wrote:
> > We return IOMAP_F_NEEDDSYNC flag from ext4_iomap_begin() for a
> > synchronous write fault when inode has some uncommitted metadata
> > changes. In the fault handler ext4_dax_fault() we then detect this case,
> > call vfs_fsync_range() to make sure all metadata is committed, and call
> > dax_insert_pfn_mkwrite() to insert page table entry. Note that this will
> > also dirty corresponding radix tree entry which is what we want -
> > fsync(2) will still provide data integrity guarantees for applications
> > not using userspace flushing. And applications using userspace flushing
> > can avoid calling fsync(2) and thus avoid the performance overhead.
> > 
> > Signed-off-by: Jan Kara <jack@suse.cz>
> > ---
> >  fs/ext4/file.c       |  6 +++++-
> >  fs/ext4/inode.c      | 15 +++++++++++++++
> >  fs/jbd2/journal.c    | 17 +++++++++++++++++
> >  include/linux/jbd2.h |  1 +
> >  4 files changed, 38 insertions(+), 1 deletion(-)
> 
> <>
> 
> > diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
> > index 31db875bc7a1..13a198924a0f 100644
> > --- a/fs/ext4/inode.c
> > +++ b/fs/ext4/inode.c
> > @@ -3394,6 +3394,19 @@ static int ext4_releasepage(struct page *page, gfp_t wait)
> >  }
> >  
> >  #ifdef CONFIG_FS_DAX
> > +static bool ext4_inode_datasync_dirty(struct inode *inode)
> > +{
> > +	journal_t *journal = EXT4_SB(inode->i_sb)->s_journal;
> > +
> > +	if (journal)
> > +		return !jbd2_transaction_committed(journal,
> > +					EXT4_I(inode)->i_datasync_tid);
> > +	/* Any metadata buffers to write? */
> > +	if (!list_empty(&inode->i_mapping->private_list))
> > +		return true;
> > +	return inode->i_state & I_DIRTY_DATASYNC;
> > +}
> 
> I just had 2 quick questions on this:
> 
> 1) Does ext4 actually use inode->i_mapping->private_list to keep track of
> dirty metadata buffers?  The comment above ext4_write_end() leads me to
> believe that this list is unused?
> 
>  * ext4 never places buffers on inode->i_mapping->private_list.  metadata
>  * buffers are managed internally.
> 
> Or does the above comment only apply to ext4 with a journal?

Yes, the above applies for ext4 with a journal. ext4 without a journal uses
inode->i_mapping->private_list for metadata tracking. And DAX can be used
without the journal just fine...

> 2) Where is I_DIRTY_DATASYNC set in inode->i_state?  I poked around a bit and
> couldn't see it.

Never directly (at least for ext4). But it will get set by
mark_inode_dirty() as I_DIRTY contains I_DIRTY_DATASYNC.

> The rest of the patch looks good to me, and you can add:
> 
> Reviewed-by: Ross Zwisler <ross.zwisler@linux.intel.com>

Thanks!

								Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR