Re: [PATCH v6 2/5] fs, xfs: introduce S_IOMAP_SEALED

[Dan Williams]

On Thu, Aug 24, 2017 at 9:13 AM, Christoph Hellwig <hch@xxxxxx> wrote:
> I'm still very unhappy about the get/set flag state.  What is the
> reason you can't use/extend leases? (take a look at the fcntl
> man page and look for Leases).  A variant of the concept is what
> the pNFS block server uses.

So I think leases could potentially be extended to replace the inode
flag. A MAP_DIRECT operation would take out a lease that is broken by
break_layouts(). However, like the pNFS case the lease break would
need to held off while any DMA might be in-flight. We can use an
elevated page count as that indication as ZONE_DEVICE pages only ever
have an elevated page count in response to get_user_pages().

However, I think the only practical difference is turning an immediate
ETXTBSY response that S_IOMAP_SEALED provides into an indefinite
blocking wait for break_layouts() to complete. Can pNFS run
break_layouts() in bounded time?

As far I can see a lease and S_IOMAP_SEALED have the same DMA
cancelling problem, so a lease is not better in that regard. Absent an
overlaying protocol like pNFS, I think S_IOMAP_SEALED is cleaner
because it fails incompatible operations outright rather than stalls
them in break_layouts(). Were their other benefits to a lease over an
inode flag that you had in mind for this case where the protocol is
userspace defined? Maybe I'm thinking too small on the ways a lease
might be extended.