Re: [RFC PATCH] NFSD: Make FILE_SYNC WRITEs comply with spec

[Christoph Hellwig <hch@infradead.org>]

On Thu, Oct 23, 2025 at 08:46:02AM -0400, Chuck Lever wrote:
> On 10/23/25 2:38 AM, Christoph Hellwig wrote:
> > On Wed, Oct 22, 2025 at 12:22:37PM -0400, Chuck Lever wrote:
> >> -		kiocb.ki_flags |= IOCB_DSYNC;
> >> +		kiocb.ki_flags |=
> >> +			(stable == NFS_FILE_SYNC ? IOCB_SYNC : IOCB_DSYNC);
> > 
> > IOCB_SYNC without IOCB_DSYNC is always wrong, see my reply to the
> > previous thread.
> > 
> 
> I see only one API consumer that sets both: NFS LOCALIO. There are not
> enough examples for me to draw any kind of conclusion, and I haven't
> found documentation for these flags, fwiw.

The primary way to set IOCB_DSYNC and IOCB_SYNC is iocb_flags() in
include/linux/fs.h, which sets IOCB_DSYNC for O_DSYNC, and IOCB_SYNC for
__O_SYNC.  O_SYNC is defined as O_DSYNC | __O_SYNC for the historic
reasons explained in the other thread.

The main consumer of IOCB_DSYNC and IOCB_SYNC is generic_write_sync in
include/linux/fs.h, which calls into vfs_fsync_range if IOCB_DSYNC is set
(or IS_SYNC() is true on the inode, but that's irrelevant here), and
only uses IOCB_SYNC to clear the datasync flag to vfs_fsync_range if
it is set, i.e. no syncing is done without IOCB_DSYNC.

As for other uses of these flags:

fuse_write_flags should be assigning __O_SYNC instead of O_SYNC for
clarify, but the code is not incorrect.
__iomap_dio_rw does the right thing in checking IOCB_SYNC for
controlling the REQ_FUA use.
netfs_perform_write should just be checking IOCB_DYSNC, but that whole
area looks bogus.  I'll follow up with Dave.
nfs_do_local_write does the right thing assuming nfs stable writes
want O_SYNC.
I don't understand what ovl_write_iter tries to do from just reading the
code.