Re: [PATCH 1/3] fs,block: Introduce IOCB_ZONE_APPEND and direct-io handling

[Kanchan Joshi <joshi.k@samsung.com>]

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On Thu, Jun 18, 2020 at 07:16:19AM +0000, Damien Le Moal wrote:
>On 2020/06/18 2:27, Kanchan Joshi wrote:
>> From: Selvakumar S <selvakuma.s1@samsung.com>
>>
>> Introduce IOCB_ZONE_APPEND flag, which is set in kiocb->ki_flags for
>> zone-append. Direct I/O submission path uses this flag to send bio with
>> append op. And completion path uses the same to return zone-relative
>> offset to upper layer.
>>
>> Signed-off-by: SelvaKumar S <selvakuma.s1@samsung.com>
>> Signed-off-by: Kanchan Joshi <joshi.k@samsung.com>
>> Signed-off-by: Nitesh Shetty <nj.shetty@samsung.com>
>> Signed-off-by: Javier Gonzalez <javier.gonz@samsung.com>
>> ---
>>  fs/block_dev.c     | 19 ++++++++++++++++++-
>>  include/linux/fs.h |  1 +
>>  2 files changed, 19 insertions(+), 1 deletion(-)
>>
>> diff --git a/fs/block_dev.c b/fs/block_dev.c
>> index 47860e5..4c84b4d0 100644
>> --- a/fs/block_dev.c
>> +++ b/fs/block_dev.c
>> @@ -185,6 +185,10 @@ static unsigned int dio_bio_write_op(struct kiocb *iocb)
>>  	/* avoid the need for a I/O completion work item */
>>  	if (iocb->ki_flags & IOCB_DSYNC)
>>  		op |= REQ_FUA;
>> +#ifdef CONFIG_BLK_DEV_ZONED
>> +	if (iocb->ki_flags & IOCB_ZONE_APPEND)
>> +		op |= REQ_OP_ZONE_APPEND | REQ_NOMERGE;
>> +#endif
>
>No need for the #ifdef. And no need for the REQ_NOMERGE either since
>REQ_OP_ZONE_APPEND requests are defined as not mergeable already.
>
>>  	return op;
>>  }
>>
>> @@ -295,6 +299,14 @@ static int blkdev_iopoll(struct kiocb *kiocb, bool wait)
>>  	return blk_poll(q, READ_ONCE(kiocb->ki_cookie), wait);
>>  }
>>
>> +#ifdef CONFIG_BLK_DEV_ZONED
>> +static inline long blkdev_bio_end_io_append(struct bio *bio)
>> +{
>> +	return (bio->bi_iter.bi_sector %
>> +		blk_queue_zone_sectors(bio->bi_disk->queue)) << SECTOR_SHIFT;
>
>A zone size is at most 4G sectors as defined by the queue chunk_sectors limit
>(unsigned int). It means that the return value here can overflow due to the
>shift, at least on 32bits arch.
>
>And as Pavel already commented, zone sizes are power of 2 so you can use
>bitmasks instead of costly divisions.
>
>> +}
>> +#endif
>> +
>>  static void blkdev_bio_end_io(struct bio *bio)
>>  {
>>  	struct blkdev_dio *dio = bio->bi_private;
>> @@ -307,15 +319,20 @@ static void blkdev_bio_end_io(struct bio *bio)
>>  		if (!dio->is_sync) {
>>  			struct kiocb *iocb = dio->iocb;
>>  			ssize_t ret;
>> +			long res = 0;
>>
>>  			if (likely(!dio->bio.bi_status)) {
>>  				ret = dio->size;
>>  				iocb->ki_pos += ret;
>> +#ifdef CONFIG_BLK_DEV_ZONED
>> +				if (iocb->ki_flags & IOCB_ZONE_APPEND)
>> +					res = blkdev_bio_end_io_append(bio);
>
>overflow... And no need for the #ifdef.
>
>> +#endif
>>  			} else {
>>  				ret = blk_status_to_errno(dio->bio.bi_status);
>>  			}
>>
>> -			dio->iocb->ki_complete(iocb, ret, 0);
>> +			dio->iocb->ki_complete(iocb, ret, res);
>
>ki_complete interface also needs to be changed to have a 64bit last argument to
>avoid overflow.
>
>And this all does not work anyway because __blkdev_direct_IO() and
>__blkdev_direct_IO_simple() both call bio_iov_iter_get_pages() *before*
>dio_bio_write_op() is called. This means that bio_iov_iter_get_pages() will not
>see that it needs to get the pages for a zone append command and so will not
>call __bio_iov_append_get_pages(). That leads to BIO split and potential
>corruption of the user data as fragments of the kiocb may get reordered.
>
>There is a lot more to do to the block_dev direct IO code for this to work.

Thanks a lot for the great review. Very helpful. We'll fix in V2. 

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