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.
