3:48 p.m.
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(a)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 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(a)suse.com>
SUSE Labs, CR