Re: [PATCH v3 14/21] iomap: Sub-extent zeroing

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

On 01/05/2024 02:07, Dave Chinner wrote:
> On Mon, Apr 29, 2024 at 05:47:39PM +0000, John Garry wrote:
>> For FS_XFLAG_FORCEALIGN support, we want to treat any sub-extent IO like
>> sub-fsblock DIO, in that we will zero the sub-extent when the mapping is
>> unwritten.
>>
>> This will be important for atomic writes support, in that atomically
>> writing over a partially written extent would mean that we would need to
>> do the unwritten extent conversion write separately, and the write could
>> no longer be atomic.
>>
>> It is the task of the FS to set iomap.extent_size per iter to indicate
>> sub-extent zeroing required.
>>
>> Signed-off-by: John Garry <john.g.garry@oracle.com>
> 
> Shouldn't this be done before the XFS feature is enabled in the
> series?

Well, it is done before XFS iomap zeroing support patch. But I can move 
this patch to the very beginning of the series.

> 
>> ---
>>   fs/iomap/direct-io.c  | 17 +++++++++++------
>>   include/linux/iomap.h |  1 +
>>   2 files changed, 12 insertions(+), 6 deletions(-)
>>
>> diff --git a/fs/iomap/direct-io.c b/fs/iomap/direct-io.c
>> index f3b43d223a46..a3ed7cfa95bc 100644
>> --- a/fs/iomap/direct-io.c
>> +++ b/fs/iomap/direct-io.c
>> @@ -277,7 +277,7 @@ static loff_t iomap_dio_bio_iter(const struct iomap_iter *iter,
>>   {
>>   	const struct iomap *iomap = &iter->iomap;
>>   	struct inode *inode = iter->inode;
>> -	unsigned int fs_block_size = i_blocksize(inode), pad;
>> +	unsigned int zeroing_size, pad;
>>   	loff_t length = iomap_length(iter);
>>   	loff_t pos = iter->pos;
>>   	blk_opf_t bio_opf;
>> @@ -288,6 +288,11 @@ static loff_t iomap_dio_bio_iter(const struct iomap_iter *iter,
>>   	size_t copied = 0;
>>   	size_t orig_count;
>>   
>> +	if (iomap->extent_size)
>> +		zeroing_size = iomap->extent_size;
>> +	else
>> +		zeroing_size = i_blocksize(inode);
> 
> Oh, the dissonance!
> 
> iomap->extent_size isn't an extent size at all.

Right, it's a poorly chosen name

> 
> The size of the extent the iomap returns is iomap->length. This new
> variable is the IO specific "block size" that should be assumed by
> the dio code to determine if padding should be done.
> 
> IOWs, I think we should add an "io_block_size" field to the iomap,
> and every filesystem that supports iomap should set it to the
> filesystem block size (i_blocksize(inode)). Then the changes to the
> iomap code end up just being:
> 
> 
> -	unsigned int fs_block_size = i_blocksize(inode), pad;
> +	unsigned int fs_block_size = iomap->io_block_size, pad;
> 
> And the patch that introduces that infrastructure change will also
> change all the filesystem implementations to unconditionally set
> iomap->io_block_size to i_blocksize().

ok

> 
> Then, in a separate patch, you can add XFS support for large IO
> block sizes when we have either a large rtextsize or extent size
> hints set.

I hadn't been considering large rtextsize for this. I suppose that it 
could be added.

> 
>> +
>>   	if ((pos | length) & (bdev_logical_block_size(iomap->bdev) - 1) ||
>>   	    !bdev_iter_is_aligned(iomap->bdev, dio->submit.iter))
>>   		return -EINVAL;
>> @@ -354,8 +359,8 @@ static loff_t iomap_dio_bio_iter(const struct iomap_iter *iter,
>>   		dio->iocb->ki_flags &= ~IOCB_HIPRI;
>>   
>>   	if (need_zeroout) {
>> -		/* zero out from the start of the block to the write offset */
>> -		pad = pos & (fs_block_size - 1);
>> +		/* zero out from the start of the region to the write offset */
>> +		pad = pos & (zeroing_size - 1);
>>   		if (pad)
>>   			iomap_dio_zero(iter, dio, pos - pad, pad);
>>   	}
>> @@ -428,10 +433,10 @@ static loff_t iomap_dio_bio_iter(const struct iomap_iter *iter,
>>   zero_tail:
>>   	if (need_zeroout ||
>>   	    ((dio->flags & IOMAP_DIO_WRITE) && pos >= i_size_read(inode))) {
>> -		/* zero out from the end of the write to the end of the block */
>> -		pad = pos & (fs_block_size - 1);
>> +		/* zero out from the end of the write to the end of the region */
>> +		pad = pos & (zeroing_size - 1);
>>   		if (pad)
>> -			iomap_dio_zero(iter, dio, pos, fs_block_size - pad);
>> +			iomap_dio_zero(iter, dio, pos, zeroing_size - pad);
>>   	}
>>   out:
>>   	/* Undo iter limitation to current extent */
>> diff --git a/include/linux/iomap.h b/include/linux/iomap.h
>> index 6fc1c858013d..42623b1cdc04 100644
>> --- a/include/linux/iomap.h
>> +++ b/include/linux/iomap.h
>> @@ -97,6 +97,7 @@ struct iomap {
>>   	u64			length;	/* length of mapping, bytes */
>>   	u16			type;	/* type of mapping */
>>   	u16			flags;	/* flags for mapping */
>> +	unsigned int		extent_size;
> 
> This needs a descriptive comment. At minimum, it should tell the
> reader what units are used for the variable.  If it is bytes, then
> it needs to be a u64, because XFS can have extent size hints well
> beyond 2^32 bytes in length.
> 

ok

Thanks,
John