On Fri, Jul 31, 2020 at 12:29 PM Damien Le Moal <Damien.LeMoal@xxxxxxx> wrote: > > On 2020/07/31 15:45, hch@xxxxxxxxxxxxx wrote: > > On Fri, Jul 31, 2020 at 06:42:10AM +0000, Damien Le Moal wrote: > >>> - We may not be able to use RWF_APPEND, and need exposing a new > >>> type/flag (RWF_INDIRECT_OFFSET etc.) user-space. Not sure if this > >>> sounds outrageous, but is it OK to have uring-only flag which can be > >>> combined with RWF_APPEND? > >> > >> Why ? Where is the problem ? O_APPEND/RWF_APPEND is currently meaningless for > >> raw block device accesses. We could certainly define a meaning for these in the > >> context of zoned block devices. > > > > We can't just add a meaning for O_APPEND on block devices now, > > as it was previously silently ignored. I also really don't think any > > of these semantics even fit the block device to start with. If you > > want to work on raw zones use zonefs, that's what is exists for. > > Which is fine with me. Just trying to say that I think this is exactly the > discussion we need to start with. What interface do we implement... > > Allowing zone append only through zonefs as the raw block device equivalent, all > the O_APPEND/RWF_APPEND semantic is defined and the "return written offset" > implementation in VFS would be common for all file systems, including regular > ones. Beside that, there is I think the question of short writes... Not sure if > short writes can currently happen with async RWF_APPEND writes to regular files. > I think not but that may depend on the FS. generic_write_check_limits (called by generic_write_checks, used by most FS) may make it short, and AFAIK it does not depend on async/sync. This was one of the reason why we chose to isolate the operation by a different IOCB flag and not by IOCB_APPEND alone. For block-device these checks are not done, but there is another place when it receives writes spanning beyond EOF and iov_iter_truncate() adjusts it before sending it down. And we return failure for that case in V4- "Ref: [PATCH v4 3/6] uio: return status with iov truncation" -- Joshi
