Re: [PATCH v3 2/2] ext4: handle layout changes to pinned DAX mappings

Dan Williams
2018-07-05T20:40:17Z

On Wed, Jul 4, 2018 at 8:59 PM, Darrick J. Wong <darrick.wong-QHcLZuEGTsvQT0dZR+AlfA@public.gmane.org> 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-VuQAYsv1563Yd54FQh9/CA@public.gmane.org>
>> > > > Reviewed-by: Jan Kara <jack-AlSwsSmVLrQ@public.gmane.org>
>> > > > Reviewed-by: Lukas Czerner <lczerner-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
>> > > > ---
>> > > >
>> > > > 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.