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

[Jan Kara <jack@suse.cz>]

On Fri 27-10-17 08:16:11, Dave Chinner wrote:
> On Thu, Oct 26, 2017 at 05:48:04PM +0200, Jan Kara wrote:
> > > > 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

Well, they all do sync regardless of who dirtied the inode on every *write*
fault.

> 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....

MAP_SYNC as I've implemented it provides guarantees only for data the
process has actually written. I agree with that and it was a conscious
decision. In my opinion that covers most usecases, provides reasonably
simple semantics (i.e., if you write data through MAP_SYNC mapping, you can
persist it just using CPU instructions), and reasonable performance.

Now you seem to suggest the semantics should be: "Data you have read from or
written to a MAP_SYNC mapping can be persisted using CPU instructions." And
from implementation POV we can do that rather easily (just rip out the
IOMAP_WRITE checks). But I'm unsure whether this additional guarantee would
be useful enough to justify the slowdown of read faults? I was not able to
come up with a good usecase and so I've decided for current semantics. What
do other people think?

And now that I've spelled out exact semantics I don't think your comparison
that you can fsync() data you didn't write quite matches - with MAP_SYNC
you will have to at least read the data to be able to persist it and you
don't have that requirement for fsync() either...

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