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@xxxxxx> > > > > > > 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@xxxxxxxxxxxxx
