On Fri, Jul 06, 2018 at 09:29:34AM +1000, Dave Chinner wrote: > On Thu, Jul 05, 2018 at 01:40:17PM -0700, Dan Williams wrote: > > On Wed, Jul 4, 2018 at 8:59 PM, Darrick J. Wong <darrick.wong@xxxxxxxxxx> wrote: > > > On Thu, Jul 05, 2018 at 09:54:14AM +1000, Dave Chinner wrote: > > >> On Wed, Jul 04, 2018 at 02:27:23PM +0200, Jan Kara wrote: > > >> > On Wed 04-07-18 10:49:23, Dave Chinner wrote: > > >> > > On Mon, Jul 02, 2018 at 11:29:12AM -0600, Ross Zwisler wrote: > > >> > > > Follow the lead of xfs_break_dax_layouts() and add synchronization between > > >> > > > operations in ext4 which remove blocks from an inode (hole punch, truncate > > >> > > > down, etc.) and pages which are pinned due to DAX DMA operations. > > >> > > > > > >> > > > Signed-off-by: Ross Zwisler <ross.zwisler@xxxxxxxxxxxxxxx> > > >> > > > Reviewed-by: Jan Kara <jack@xxxxxxx> > > >> > > > Reviewed-by: Lukas Czerner <lczerner@xxxxxxxxxx> > > >> > > > --- > > >> > > > > > >> > > > Changes since v2: > > >> > > > * Added a comment to ext4_insert_range() explaining why we don't call > > >> > > > ext4_break_layouts(). (Jan) > > >> > > > > >> > > Which I think is wrong and will cause data corruption. > > >> > > > > >> > > > @@ -5651,6 +5663,11 @@ int ext4_insert_range(struct inode *inode, loff_t offset, loff_t len) > > >> > > > LLONG_MAX); > > >> > > > if (ret) > > >> > > > goto out_mmap; > > >> > > > + /* > > >> > > > + * We don't need to call ext4_break_layouts() because we aren't > > >> > > > + * removing any blocks from the inode. We are just changing their > > >> > > > + * offset by inserting a hole. > > >> > > > + */ > > > > > > Does calling ext4_break_layouts from insert range not work? > > > > > > It's my understanding that file leases work are a mechanism for the > > > filesystem to delegate some of its authority over physical space > > > mappings to "client" software. AFAICT it's used for mmap'ing pmem > > > directly into userspace and exporting space on shared storage over > > > pNFS. Some day we might use the same mechanism for the similar things > > > that RDMA does, or the swapfile code since that's essentially how it > > > works today. > > > > > > The other part of these file leases is that the filesystem revokes them > > > any time it wants to perform a mapping operation on a file. This breaks > > > my mental model of how leases work, and if you commit to this for ext4 > > > then I'll have to remember that leases are different between xfs and > > > ext4. Worse, since the reason for skipping ext4_break_layouts seems to > > > be the implementation detail that "DAX won't care", then someone else > > > wiring up pNFS/future RDMA/whatever will also have to remember to put it > > > back into ext4 or else kaboom. > > > > > > Granted, Dave said all these things already, but I actually feel > > > strongly enough to reiterate. > > > > This patch kit is only for the DAX fix, this isn't full layout lease > > support. Even XFS is special casing unmap with the BREAK_UNMAP flag. > > So ext4 is achieving feature parity for BREAK_UNMAP, just not > > BREAK_WRITE, yet. > > BREAK_UNMAP is issued unconditionally by XFS for all fallocate > operations. There is no special except for extent shifting (up or > down) in XFS as this patch set is making for ext4. IOWs, this > patchset does not implement BREAK_UNMAP with the same semantics as > XFS. If anything this is very usefull discussion ( at least for me ) and what I do take away from it is that there is no documentation, nor specification of the leases nor BREAK_UNMAP nor BREAK_WRITE. grep -iR -e break_layout -e BREAK_UNMAP -e BREAK_WRITE Documentation/* Maybe someone with a good understanding of how this stuff is supposed to be done could write it down so filesystem devs can make it behave the same. Thanks! -Lukas > > Cheers, > > Dave. > -- > Dave Chinner > david@xxxxxxxxxxxxx
