On Thu, Aug 24, 2017 at 11:00 PM, Dan Williams <dan.j.williams@xxxxxxxxx> wrote: > 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. At a minimum I can at least use a new lease type as an indication of when to bail out an block-map operation with ETXTBSY, and reuse the lease security model. That way we at least start to converge the in-kernel lease machinery for pinning blocks with this userspace mechanism. -- To unsubscribe from this list: send the line "unsubscribe linux-api" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
