[Samba] Direct I/O

[Christof Schmitt]

On Tue, Jan 25, 2022 at 12:45:07PM -0800, Jeremy Allison wrote:
> On Tue, Jan 25, 2022 at 09:09:36PM +0100, Ralph Boehme wrote:
> > On 1/25/22 01:50, Christof Schmitt wrote:
> > > FWIW, preadv2() has RWF_DSYNC and RWF_SYNC which provide O_DSYNC and
> > > O_SYNC semantics per write. Maybe one of those aligns more closely with
> > > WRITE_UNBUFFERED.
> > 
> > perfect! Any takers? :)

Strictly speaking, WRITE_THROUGH would match the SYNC flag (write the
data to stable storage). WRITE_UNBUFFERED implies writing to storage,
but ensure that there is no data buffered, which has no Linux/POSIX
equivalent call (to my knowledge). So smbd sets write_through for both
of the flags.

> Hmmm. How to add the in_flags field for the SMB_VFS_PREAD_SEND/SMB_VFS_PWRITE_SEND ?
> 
> Both take an fsp, so is this something we should be adding
> to the files_struct as a structure element (or a one element
> in the fsp_flags bitfield) or as a separate 'flags' parameter
> to the VFS calls ?
> 
> Personally I prefer adding to the files_struct as that way it's
> available to all handle-based VFS calls and we don't have
> to change SMB_VFS_PREAD_SEND/SMB_VFS_PWRITE_SEND, and can
> just make use of the feature inside vfs_default.c on
> platforms that have it.

This flag is specific for the individual write request (passed through
smbd_smb2_write_state to schedule_aio_smb2_write). No other operation
than the write has the flag, so having it in files_struct seems a bit
misplaced. Maybe adding a flags field to pwrite_send would be better?

Christof