[v3] ext4: Add atomic writes support for DIO

[PATCH v3 4/4] ext4: Do not fallback to buffered-io for DIO atomic write

Posted by Ritesh Harjani (IBM) 3 weeks, 4 days ago

atomic writes is currently only supported for single fsblock and only
for direct-io. We should not return -ENOTBLK for atomic writes since we
want the atomic write request to either complete fully or fail
otherwise. We should not fallback to buffered-io in case of DIO atomic
write requests.
Let's also catch if this ever happens by adding some WARN_ON_ONCE before
buffered-io handling for direct-io atomic writes.

More details of the discussion [1].

[1]: https://lore.kernel.org/linux-xfs/cover.1729825985.git.ritesh.list@gmail.com/T/#m9dbecc11bed713ed0d7a486432c56b105b555f04

Signed-off-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
---
 fs/ext4/file.c  |  7 +++++++
 fs/ext4/inode.c | 14 +++++++++-----
 2 files changed, 16 insertions(+), 5 deletions(-)

diff --git a/fs/ext4/file.c b/fs/ext4/file.c
index 8116bd78910b..61787a37e9d4 100644
--- a/fs/ext4/file.c
+++ b/fs/ext4/file.c
@@ -599,6 +599,13 @@ static ssize_t ext4_dio_write_iter(struct kiocb *iocb, struct iov_iter *from)
 		ssize_t err;
 		loff_t endbyte;
 
+		/*
+		 * There is no support for atomic writes on buffered-io yet,
+		 * we should never fallback to buffered-io for DIO atomic
+		 * writes.
+		 */
+		WARN_ON_ONCE(iocb->ki_flags & IOCB_ATOMIC);
+
 		offset = iocb->ki_pos;
 		err = ext4_buffered_write_iter(iocb, from);
 		if (err < 0)
diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index fcdee27b9aa2..26b3c84d7f64 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -3449,12 +3449,16 @@ static int ext4_iomap_end(struct inode *inode, loff_t offset, loff_t length,
 {
 	/*
 	 * Check to see whether an error occurred while writing out the data to
-	 * the allocated blocks. If so, return the magic error code so that we
-	 * fallback to buffered I/O and attempt to complete the remainder of
-	 * the I/O. Any blocks that may have been allocated in preparation for
-	 * the direct I/O will be reused during buffered I/O.
+	 * the allocated blocks. If so, return the magic error code for
+	 * non-atomic write so that we fallback to buffered I/O and attempt to
+	 * complete the remainder of the I/O.
+	 * For atomic writes we will simply fail the I/O request if we coudn't
+	 * write anything. For non-atomic writes, any blocks that may have been
+	 * allocated in preparation for the direct I/O will be reused during
+	 * buffered I/O.
 	 */
-	if (flags & (IOMAP_WRITE | IOMAP_DIRECT) && written == 0)
+	if (!(flags & IOMAP_ATOMIC) && (flags & (IOMAP_WRITE | IOMAP_DIRECT))
+			&& written == 0)
 		return -ENOTBLK;
 
 	return 0;
-- 
2.46.0

Re: [PATCH v3 4/4] ext4: Do not fallback to buffered-io for DIO atomic write

Posted by Darrick J. Wong 3 weeks, 3 days ago

On Wed, Oct 30, 2024 at 09:27:41PM +0530, Ritesh Harjani (IBM) wrote:
> atomic writes is currently only supported for single fsblock and only
> for direct-io. We should not return -ENOTBLK for atomic writes since we
> want the atomic write request to either complete fully or fail
> otherwise. We should not fallback to buffered-io in case of DIO atomic
> write requests.
> Let's also catch if this ever happens by adding some WARN_ON_ONCE before
> buffered-io handling for direct-io atomic writes.
> 
> More details of the discussion [1].
> 
> [1]: https://lore.kernel.org/linux-xfs/cover.1729825985.git.ritesh.list@gmail.com/T/#m9dbecc11bed713ed0d7a486432c56b105b555f04
> 
> Signed-off-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
> ---
>  fs/ext4/file.c  |  7 +++++++
>  fs/ext4/inode.c | 14 +++++++++-----
>  2 files changed, 16 insertions(+), 5 deletions(-)
> 
> diff --git a/fs/ext4/file.c b/fs/ext4/file.c
> index 8116bd78910b..61787a37e9d4 100644
> --- a/fs/ext4/file.c
> +++ b/fs/ext4/file.c
> @@ -599,6 +599,13 @@ static ssize_t ext4_dio_write_iter(struct kiocb *iocb, struct iov_iter *from)
>  		ssize_t err;
>  		loff_t endbyte;
>  
> +		/*
> +		 * There is no support for atomic writes on buffered-io yet,
> +		 * we should never fallback to buffered-io for DIO atomic
> +		 * writes.
> +		 */
> +		WARN_ON_ONCE(iocb->ki_flags & IOCB_ATOMIC);
> +
>  		offset = iocb->ki_pos;
>  		err = ext4_buffered_write_iter(iocb, from);
>  		if (err < 0)
> diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
> index fcdee27b9aa2..26b3c84d7f64 100644
> --- a/fs/ext4/inode.c
> +++ b/fs/ext4/inode.c
> @@ -3449,12 +3449,16 @@ static int ext4_iomap_end(struct inode *inode, loff_t offset, loff_t length,
>  {
>  	/*
>  	 * Check to see whether an error occurred while writing out the data to
> -	 * the allocated blocks. If so, return the magic error code so that we
> -	 * fallback to buffered I/O and attempt to complete the remainder of
> -	 * the I/O. Any blocks that may have been allocated in preparation for
> -	 * the direct I/O will be reused during buffered I/O.
> +	 * the allocated blocks. If so, return the magic error code for
> +	 * non-atomic write so that we fallback to buffered I/O and attempt to
> +	 * complete the remainder of the I/O.
> +	 * For atomic writes we will simply fail the I/O request if we coudn't
> +	 * write anything. For non-atomic writes, any blocks that may have been
> +	 * allocated in preparation for the direct I/O will be reused during
> +	 * buffered I/O.
>  	 */
> -	if (flags & (IOMAP_WRITE | IOMAP_DIRECT) && written == 0)
> +	if (!(flags & IOMAP_ATOMIC) && (flags & (IOMAP_WRITE | IOMAP_DIRECT))

Huh.  The WRITE|DIRECT check doesn't look right to me, because the
expression returns true for any write or any directio.  I think that's
currently "ok" because ext4_iomap_end is only called for directio
writes, but this bugs me anyway.  For a directio write fallback, that
comparison really should be:

	(flags & (WRITE|DIRECT)) == (WRITE|DIRECT)

static inline bool
ext4_want_directio_fallback(unsigned flags, ssize_t written)
{
	/* must be a directio to fall back to buffered */
	if (flags & (IOMAP_WRITE | IOMAP_DIRECT)) !=
		    (IOMAP_WRITE | IOMAP_DIRECT)
		return false;

	/* atomic writes are all-or-nothing */
	if (flags & IOMAP_ATOMIC)
		return false;

	/* can only try again if we wrote nothing */
	return written == 0;
}

	if (ext4_want_directio_fallback(flags, written))
		return -ENOTBLK;

> +			&& written == 0)

Nit: put the '&&' operator on the previous line when there's a multiline
expression.

--D

>  		return -ENOTBLK;
>  
>  	return 0;
> -- 
> 2.46.0
> 
>

Re: [PATCH v3 4/4] ext4: Do not fallback to buffered-io for DIO atomic write

Posted by Ritesh Harjani (IBM) 3 weeks, 2 days ago

"Darrick J. Wong" <djwong@kernel.org> writes:

> On Wed, Oct 30, 2024 at 09:27:41PM +0530, Ritesh Harjani (IBM) wrote:
>> atomic writes is currently only supported for single fsblock and only
>> for direct-io. We should not return -ENOTBLK for atomic writes since we
>> want the atomic write request to either complete fully or fail
>> otherwise. We should not fallback to buffered-io in case of DIO atomic
>> write requests.
>> Let's also catch if this ever happens by adding some WARN_ON_ONCE before
>> buffered-io handling for direct-io atomic writes.
>> 
>> More details of the discussion [1].
>> 
>> [1]: https://lore.kernel.org/linux-xfs/cover.1729825985.git.ritesh.list@gmail.com/T/#m9dbecc11bed713ed0d7a486432c56b105b555f04
>> 
>> Signed-off-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
>> ---
>>  fs/ext4/file.c  |  7 +++++++
>>  fs/ext4/inode.c | 14 +++++++++-----
>>  2 files changed, 16 insertions(+), 5 deletions(-)
>> 
>> diff --git a/fs/ext4/file.c b/fs/ext4/file.c
>> index 8116bd78910b..61787a37e9d4 100644
>> --- a/fs/ext4/file.c
>> +++ b/fs/ext4/file.c
>> @@ -599,6 +599,13 @@ static ssize_t ext4_dio_write_iter(struct kiocb *iocb, struct iov_iter *from)
>>  		ssize_t err;
>>  		loff_t endbyte;
>>  
>> +		/*
>> +		 * There is no support for atomic writes on buffered-io yet,
>> +		 * we should never fallback to buffered-io for DIO atomic
>> +		 * writes.
>> +		 */
>> +		WARN_ON_ONCE(iocb->ki_flags & IOCB_ATOMIC);
>> +
>>  		offset = iocb->ki_pos;
>>  		err = ext4_buffered_write_iter(iocb, from);
>>  		if (err < 0)
>> diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
>> index fcdee27b9aa2..26b3c84d7f64 100644
>> --- a/fs/ext4/inode.c
>> +++ b/fs/ext4/inode.c
>> @@ -3449,12 +3449,16 @@ static int ext4_iomap_end(struct inode *inode, loff_t offset, loff_t length,
>>  {
>>  	/*
>>  	 * Check to see whether an error occurred while writing out the data to
>> -	 * the allocated blocks. If so, return the magic error code so that we
>> -	 * fallback to buffered I/O and attempt to complete the remainder of
>> -	 * the I/O. Any blocks that may have been allocated in preparation for
>> -	 * the direct I/O will be reused during buffered I/O.
>> +	 * the allocated blocks. If so, return the magic error code for
>> +	 * non-atomic write so that we fallback to buffered I/O and attempt to
>> +	 * complete the remainder of the I/O.
>> +	 * For atomic writes we will simply fail the I/O request if we coudn't
>> +	 * write anything. For non-atomic writes, any blocks that may have been
>> +	 * allocated in preparation for the direct I/O will be reused during
>> +	 * buffered I/O.
>>  	 */
>> -	if (flags & (IOMAP_WRITE | IOMAP_DIRECT) && written == 0)
>> +	if (!(flags & IOMAP_ATOMIC) && (flags & (IOMAP_WRITE | IOMAP_DIRECT))
>
> Huh.  The WRITE|DIRECT check doesn't look right to me, because the
> expression returns true for any write or any directio.  I think that's
> currently "ok" because ext4_iomap_end is only called for directio
> writes, but this bugs me anyway.  For a directio write fallback, that
> comparison really should be:
>
> 	(flags & (WRITE|DIRECT)) == (WRITE|DIRECT)
>

yes. You are right. It is working since ext4 only supports iomap
for DIRECTIO. But I agree it's better be fixed to avoid problem in future.

> static inline bool
> ext4_want_directio_fallback(unsigned flags, ssize_t written)
> {
> 	/* must be a directio to fall back to buffered */
> 	if (flags & (IOMAP_WRITE | IOMAP_DIRECT)) !=
> 		    (IOMAP_WRITE | IOMAP_DIRECT)
> 		return false;
>
> 	/* atomic writes are all-or-nothing */
> 	if (flags & IOMAP_ATOMIC)
> 		return false;
>
> 	/* can only try again if we wrote nothing */
> 	return written == 0;
> }
>
> 	if (ext4_want_directio_fallback(flags, written))
> 		return -ENOTBLK;
>

I like the above helper. Thanks for doing that. 
I will incorporate this in v4.


>> +			&& written == 0)
>
> Nit: put the '&&' operator on the previous line when there's a multiline
> expression.
>

I guess we don't need this if we do it with your above inline helper.
But sure, next time will keep in mind for any such changes.

> --D
>

Thanks for the review!
-ritesh

>>  		return -ENOTBLK;
>>  
>>  	return 0;
>> -- 
>> 2.46.0
>> 
>>

[PATCH v3 1/4] ext4: Add statx support for atomic writes
[PATCH v3 2/4] ext4: Check for atomic writes support in write iter
[PATCH v3 3/4] ext4: Support setting FMODE_CAN_ATOMIC_WRITE
[PATCH v3 4/4] ext4: Do not fallback to buffered-io for DIO atomic write