From: Dave Chinner Subject: Re: [PATCH 17/17] xfs: support for synchronous DAX faults Date: Fri, 27 Oct 2017 08:16:11 +1100 Message-ID: <20171026211611.GC3666@dastard> References: <20171024152415.22864-1-jack@suse.cz> <20171024152415.22864-18-jack@suse.cz> <20171024222322.GX3666@dastard> <20171026154804.GF31161@quack2.suse.cz> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Dan Williams , Ross Zwisler , Christoph Hellwig , linux-ext4-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, linux-nvdimm-hn68Rpc1hR1g9hUCZPvPmw@public.gmane.org, linux-fsdevel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, linux-xfs-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, linux-api-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, linux-mm-Bw31MaZKKs3YtjvyW6yDsg@public.gmane.org, Christoph Hellwig To: Jan Kara Return-path: Content-Disposition: inline In-Reply-To: <20171026154804.GF31161-4I4JzKEfoa/jFM9bn6wA6Q@public.gmane.org> Sender: linux-api-owner-u79uwXL29TY76Z2rM5mHXA@public.gmane.org List-Id: linux-ext4.vger.kernel.org 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 > > > > > > 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-FqsqvQoI3Ljby3iVrkZq2A@public.gmane.org