Re: [PATCH 17/17] xfs: support for synchronous DAX faults

[Jan Kara <jack-AlSwsSmVLrQ@public.gmane.org>]

On Wed 25-10-17 09:23:22, Dave Chinner wrote:
> On Tue, Oct 24, 2017 at 05:24:14PM +0200, Jan Kara wrote:
> > From: Christoph Hellwig <hch-jcswGhMUV9g@public.gmane.org>
> > 
> > Return IOMAP_F_DIRTY from xfs_file_iomap_begin() when asked to prepare
> > blocks for writing and the inode is pinned, and has dirty fields other
> > than the timestamps.
> 
> That's "fdatasync dirty", not "fsync dirty".

Correct.

> IOMAP_F_DIRTY needs a far better description of it's semantics than
> "/* block mapping is not yet on persistent storage */" so we know
> exactly what filesystems are supposed to be implementing here. I
> suspect that what it really is meant to say is:
> 
> /*
>  * IOMAP_F_DIRTY indicates the inode has uncommitted metadata to
>  * written data and requires fdatasync to commit to persistent storage.
>  */

I'll update the comment. Thanks!

> [....]
> 
> > diff --git a/fs/xfs/xfs_iomap.c b/fs/xfs/xfs_iomap.c
> > index f179bdf1644d..b43be199fbdf 100644
> > --- a/fs/xfs/xfs_iomap.c
> > +++ b/fs/xfs/xfs_iomap.c
> > @@ -33,6 +33,7 @@
> >  #include "xfs_error.h"
> >  #include "xfs_trans.h"
> >  #include "xfs_trans_space.h"
> > +#include "xfs_inode_item.h"
> >  #include "xfs_iomap.h"
> >  #include "xfs_trace.h"
> >  #include "xfs_icache.h"
> > @@ -1086,6 +1087,10 @@ xfs_file_iomap_begin(
> >  		trace_xfs_iomap_found(ip, offset, length, 0, &imap);
> >  	}
> >  
> > +	if ((flags & IOMAP_WRITE) && xfs_ipincount(ip) &&
> > +	    (ip->i_itemp->ili_fsync_fields & ~XFS_ILOG_TIMESTAMP))
> > +		iomap->flags |= IOMAP_F_DIRTY;
> 
> This is the very definition of an inode that is "fdatasync dirty".
> 
> Hmmmm, shouldn't this also be set for read faults, too?

No, read faults don't need to set IOMAP_F_DIRTY since user cannot write any
data to the page which he'd then like to be persistent. The only reason why
I thought it could be useful for a while was that it would be nice to make
MAP_SYNC mapping provide the guarantee that data you see now is the data
you'll see after a crash but we cannot provide that guarantee for RO
mapping anyway if someone else has the page mapped as well. So I just
decided not to return IOMAP_F_DIRTY for read faults.

But now that I look at XFS implementation again, it misses handling
of VM_FAULT_NEEDSYNC in xfs_filemap_pfn_mkwrite() (ext4 gets this right).
I'll fix this by using __xfs_filemap_fault() for xfs_filemap_pfn_mkwrite()
as well since it mostly duplicates it anyway... Thanks for inquiring!

								Honza
-- 
Jan Kara <jack-IBi9RG/b67k@public.gmane.org>
SUSE Labs, CR