linux-ext4 - Re: [PATCH 17/17] xfs: support for synchronous DAX faults

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Date:   Wed, 25 Oct 2017 09:23:22 +1100
From:   Dave Chinner <david@...morbit.com>
To:     Jan Kara <jack@...e.cz>
Cc:     Dan Williams <dan.j.williams@...el.com>,
        Ross Zwisler <ross.zwisler@...ux.intel.com>,
        Christoph Hellwig <hch@...radead.org>,
        linux-ext4@...r.kernel.org, linux-nvdimm@...ts.01.org,
        linux-fsdevel@...r.kernel.org, linux-xfs@...r.kernel.org,
        linux-api@...r.kernel.org, linux-mm@...ck.org,
        Christoph Hellwig <hch@....de>
Subject: Re: [PATCH 17/17] xfs: support for synchronous DAX faults

On Tue, Oct 24, 2017 at 05:24:14PM +0200, Jan Kara wrote:
> From: Christoph Hellwig <hch@....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".

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.
 */

[....]

> 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?

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com