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

[John Garry <john.g.garry@oracle.com>]

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