Re: [PATCH 1/4] iomap: add IOMAP_DIO_FSBLOCK_ALIGNED flag

Qu Wenruo
2025-10-27T21:41:30Z

在 2025/10/28 02:40, Darrick J. Wong 写道:
> On Thu, Oct 23, 2025 at 03:55:42PM +0200, Christoph Hellwig wrote:
>> From: Qu Wenruo <wqu@suse.com>
>>
>> Btrfs requires all of its bios to be fs block aligned, normally it's
>> totally fine but with the incoming block size larger than page size
>> (bs > ps) support, the requirement is no longer met for direct IOs.
>>
>> Because iomap_dio_bio_iter() calls bio_iov_iter_get_pages(), only
>> requiring alignment to be bdev_logical_block_size().
>>
>> In the real world that value is either 512 or 4K, on 4K page sized
>> systems it means bio_iov_iter_get_pages() can break the bio at any page
>> boundary, breaking btrfs' requirement for bs > ps cases.
>>
>> To address this problem, introduce a new public iomap dio flag,
>> IOMAP_DIO_FSBLOCK_ALIGNED.
>>
>> When calling __iomap_dio_rw() with that new flag, iomap_dio::flags will
>> inherit that new flag, and iomap_dio_bio_iter() will take fs block size
>> into the calculation of the alignment, and pass the alignment to
>> bio_iov_iter_get_pages(), respecting the fs block size requirement.
>>
>> The initial user of this flag will be btrfs, which needs to calculate the
>> checksum for direct read and thus requires the biovec to be fs block
>> aligned for the incoming bs > ps support.
>>
>> Signed-off-by: Qu Wenruo <wqu@suse.com>
>> Reviewed-by: Pankaj Raghav <p.raghav@samsung.com>
>> Signed-off-by: Christoph Hellwig <hch@lst.de>
>> ---
>> fs/iomap/direct-io.c | 13 ++++++++++++-
>> include/linux/iomap.h | 8 ++++++++
>> 2 files changed, 20 insertions(+), 1 deletion(-)
>>
>> diff --git a/fs/iomap/direct-io.c b/fs/iomap/direct-io.c
>> index 5d5d63efbd57..ce9cbd2bace0 100644
>> --- a/fs/iomap/direct-io.c
>> +++ b/fs/iomap/direct-io.c
>> @@ -336,10 +336,18 @@ static int iomap_dio_bio_iter(struct iomap_iter *iter, struct iomap_dio *dio)
>> int nr_pages, ret = 0;
>> u64 copied = 0;
>> size_t orig_count;
>> + unsigned int alignment = bdev_logical_block_size(iomap->bdev);
>>
>> if ((pos | length) & (bdev_logical_block_size(iomap->bdev) - 1))
>> return -EINVAL;
>>
>> + /*
>> + * Align to the larger one of bdev and fs block size, to meet the
>> + * alignment requirement of both layers.
>> + */
>> + if (dio->flags & IOMAP_DIO_FSBLOCK_ALIGNED)
>> + alignment = max(alignment, fs_block_size);
>> +
>> if (dio->flags & IOMAP_DIO_WRITE) {
>> bio_opf |= REQ_OP_WRITE;
>>
>> @@ -434,7 +442,7 @@ static int iomap_dio_bio_iter(struct iomap_iter *iter, struct iomap_dio *dio)
>> bio->bi_end_io = iomap_dio_bio_end_io;
>>
>> ret = bio_iov_iter_get_pages(bio, dio->submit.iter,
>> - bdev_logical_block_size(iomap->bdev) - 1);
>> + alignment - 1);
>> if (unlikely(ret)) {
>> /*
>> * We have to stop part way through an IO. We must fall
>> @@ -639,6 +647,9 @@ __iomap_dio_rw(struct kiocb *iocb, struct iov_iter *iter,
>> if (iocb->ki_flags & IOCB_NOWAIT)
>> iomi.flags |= IOMAP_NOWAIT;
>>
>> + if (dio_flags & IOMAP_DIO_FSBLOCK_ALIGNED)
>> + dio->flags |= IOMAP_DIO_FSBLOCK_ALIGNED;
>> +
>> if (iov_iter_rw(iter) == READ) {
>> /* reads can always complete inline */
>> dio->flags |= IOMAP_DIO_INLINE_COMP;
>> diff --git a/include/linux/iomap.h b/include/linux/iomap.h
>> index 73dceabc21c8..4da13fe24ce8 100644
>> --- a/include/linux/iomap.h
>> +++ b/include/linux/iomap.h
>> @@ -518,6 +518,14 @@ struct iomap_dio_ops {
>> */
>> #define IOMAP_DIO_PARTIAL (1 << 2)
>>
>> +/*
>> + * Ensure each bio is aligned to fs block size.
>> + *
>> + * For filesystems which need to calculate/verify the checksum of each fs
>> + * block. Otherwise they may not be able to handle unaligned bios.
>> + */
>> +#define IOMAP_DIO_FSBLOCK_ALIGNED (1 << 3)
>
> A new flag requires an update to IOMAP_F_FLAGS_STRINGS in trace.h for
> tracepoint decoding.

I'm wondering who should fix this part.

The original author (myself) or Christoph?

Thanks,
Qu

>
> The rest of the changes look ok to me, modulo hch's subsequent fixups.
>
> --D
>
>> ssize_t iomap_dio_rw(struct kiocb *iocb, struct iov_iter *iter,
>> const struct iomap_ops *ops, const struct iomap_dio_ops *dops,
>> unsigned int dio_flags, void *private, size_t done_before);
>> --
>> 2.47.3
>>
>>