Re: [PATCH v6 03/13] iomap: rework IOMAP atomic flags

[John Garry]

On 17/03/2025 13:44, Ritesh Harjani (IBM) wrote:
> >   	if (flags & IOMAP_DAX)
> >   		iomap->dax_dev = EXT4_SB(inode->i_sb)->s_daxdev;
> >   	else
> > @@ -3467,7 +3470,7 @@ static inline bool ext4_want_directio_fallback(unsigned flags, ssize_t written)
> >   		return false;
> >   
> >   	/* atomic writes are all-or-nothing */
> > -	if (flags & IOMAP_ATOMIC_HW)
> > +	if (flags & IOMAP_ATOMIC)
> >   		return false;
> >   
> The changes in ext4 is mostly straight forward. Essentially for
> an IOMAP_ATOMIC write requests we are always setting IOMAP_F_ATOMIC_BIO in
> the ->iomap_begin() routine. This is done to inform the iomap that this
> write request needs to issue an atomic bio, so iomap then goes and sets
> REQ_ATOMIC flag in the bio.

Right

> >   	/* can only try again if we wrote nothing */
> > diff --git a/fs/iomap/direct-io.c b/fs/iomap/direct-io.c
> > index 9d72b99cb447..c28685fd3362 100644
> > --- a/fs/iomap/direct-io.c
> > +++ b/fs/iomap/direct-io.c
> > @@ -349,7 +349,7 @@ static int iomap_dio_bio_iter(struct iomap_iter *iter, struct iomap_dio *dio)
> >   	if (dio->flags & IOMAP_DIO_WRITE) {
> >   		bio_opf |= REQ_OP_WRITE;
> >   
> > -		if (iter->flags & IOMAP_ATOMIC_HW) {
> > +		if (iomap->flags & IOMAP_F_ATOMIC_BIO) {
> >   			/*
> >   			* Ensure that the mapping covers the full write length,
> >   			* otherwise we will submit multiple BIOs, which is
> > @@ -677,10 +677,8 @@ __iomap_dio_rw(struct kiocb *iocb, struct iov_iter *iter,
> >   			iomi.flags |= IOMAP_OVERWRITE_ONLY;
> >   		}
> >   
> > -		if (dio_flags & IOMAP_DIO_ATOMIC_SW)
> > -			iomi.flags |= IOMAP_ATOMIC_SW;
> > -		else if (iocb->ki_flags & IOCB_ATOMIC)
> > -			iomi.flags |= IOMAP_ATOMIC_HW;
> > +		if (iocb->ki_flags & IOCB_ATOMIC)
> > +			iomi.flags |= IOMAP_ATOMIC;
> >   
> >   		/* for data sync or sync, we need sync completion processing */
> >   		if (iocb_is_dsync(iocb)) {
> > diff --git a/fs/iomap/trace.h b/fs/iomap/trace.h
> > index 69af89044ebd..9eab2c8ac3c5 100644
> > --- a/fs/iomap/trace.h
> > +++ b/fs/iomap/trace.h
> > @@ -99,7 +99,7 @@ DEFINE_RANGE_EVENT(iomap_dio_rw_queued);
> >   	{ IOMAP_FAULT,		"FAULT" }, \
> >   	{ IOMAP_DIRECT,		"DIRECT" }, \
> >   	{ IOMAP_NOWAIT,		"NOWAIT" }, \
> > -	{ IOMAP_ATOMIC_HW,	"ATOMIC_HW" }
> > +	{ IOMAP_ATOMIC,		"ATOMIC" }
> >   
> >   #define IOMAP_F_FLAGS_STRINGS \
> >   	{ IOMAP_F_NEW,		"NEW" }, \
> > diff --git a/fs/xfs/xfs_iomap.c b/fs/xfs/xfs_iomap.c
> > index 30e257f683bb..9a22ecd794eb 100644
> > --- a/fs/xfs/xfs_iomap.c
> > +++ b/fs/xfs/xfs_iomap.c
> > @@ -831,6 +831,9 @@ xfs_direct_write_iomap_begin(
> >   	if (offset + length > i_size_read(inode))
> >   		iomap_flags |= IOMAP_F_DIRTY;
> >   
> > +	if (flags & IOMAP_ATOMIC)
> > +		iomap_flags |= IOMAP_F_ATOMIC_BIO;
> > +
> >   	/*
> >   	 * COW writes may allocate delalloc space or convert unwritten COW
> >   	 * extents, so we need to make sure to take the lock exclusively here.
> > diff --git a/include/linux/iomap.h b/include/linux/iomap.h
> > index 9cd93530013c..51f4c13bd17a 100644
> > --- a/include/linux/iomap.h
> > +++ b/include/linux/iomap.h
> > @@ -60,6 +60,9 @@ struct vm_fault;
> >    * IOMAP_F_ANON_WRITE indicates that (write) I/O does not have a target block
> >    * assigned to it yet and the file system will do that in the bio submission
> >    * handler, splitting the I/O as needed.
> > + *
> > + * IOMAP_F_ATOMIC_BIO indicates that (write) I/O needs to be issued as an
> > + * atomic bio, i.e. set REQ_ATOMIC.
> >    */
>
> Maybe we can be more explicit here?
>
> IOMAP_F_ATOMIC_BIO flag indicates that write I/O must be issued as an
> atomic bio by setting the REQ_ATOMIC flag. Filesystems need to set this
> flag to inform iomap that the write I/O operation should be submitted as
> an atomic bio.

The comment for all these flags is that they should be set by the FS:

"Flags reported by the file system from iomap_begin"

So the second sentence seems to just repeat what is already said.


> This definition (or whatever you feel is the better version), should also
> go in Documentation/filesystems/iomap/design.rst

Yes, I need to update that again

> >   #define IOMAP_F_NEW		(1U << 0)
> >   #define IOMAP_F_DIRTY		(1U << 1)
> > @@ -73,6 +76,7 @@ struct vm_fault;
> >   #define IOMAP_F_XATTR		(1U << 5)
> >   #define IOMAP_F_BOUNDARY	(1U << 6)
> >   #define IOMAP_F_ANON_WRITE	(1U << 7)
> > +#define IOMAP_F_ATOMIC_BIO	(1U << 8)
> >   
> >   /*
> >    * Flags set by the core iomap code during operations:
> > @@ -189,9 +193,8 @@ struct iomap_folio_ops {
> >   #else
> >   #define IOMAP_DAX		0
> >   #endif /* CONFIG_FS_DAX */
> > -#define IOMAP_ATOMIC_HW		(1 << 9) /* HW-based torn-write protection */
> > +#define IOMAP_ATOMIC		(1 << 9) /* torn-write protection */
> >   #define IOMAP_DONTCACHE		(1 << 10)
> > -#define IOMAP_ATOMIC_SW		(1 << 11)/* SW-based torn-write protection */
> Now that we are killing separate IOMAP_ATOMIC_** names, we may would
> like to update the iomap design document as well. Otherwise it will
> carry use of IOMAP_ATOMIC_HW & IOMAP_ATOMIC_SW definitions. Instead we
> should only keep IOMAP_ATOMIC and update the design info there.

Yes, I will update it.

Thanks for the reminder.

John