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

[Dave Chinner <david@fromorbit.com>]

On Thu, Oct 26, 2017 at 05:48:04PM +0200, Jan Kara wrote:
> 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@lst.de>
> > > 
> > > 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

Isn't that the entire point of MAP_SYNC? i.e. That when we return
from a page fault, the app knows that the data and it's underlying
extent is on persistent storage?

> 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.

If there are multiple MAP_SYNC mappings to the inode, I would have
expected that they all sync all of the data/metadata on every page
fault, regardless of who dirtied the inode. An RO mapping doesn't
mean the data/metadata on the inode can't change, it just means it
can't change through that mapping.  Running fsync() to guarantee the
persistence of that data/metadata doesn't actually changing any
data....

IOWs, if read faults don't guarantee the mapped range has stable
extents on a MAP_SYNC mapping, then I think MAP_SYNC is broken
because it's not giving consistent guarantees to userspace. Yes, it
works fine when only one MAP_SYNC mapping is modifying the inode,
but the moment we have concurrent operations on the inode that
aren't MAP_SYNC or O_SYNC this goes out the window....

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com

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