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Date: Wed, 1 May 2024 11:23:14 +0100
From: John Garry <john.g.garry@...cle.com>
To: Dave Chinner <david@...morbit.com>
Cc: djwong@...nel.org, hch@....de, viro@...iv.linux.org.uk, brauner@...nel.org,
jack@...e.cz, chandan.babu@...cle.com, willy@...radead.org,
axboe@...nel.dk, martin.petersen@...cle.com,
linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
tytso@....edu, jbongio@...gle.com, ojaswin@...ux.ibm.com,
ritesh.list@...il.com, mcgrof@...nel.org, p.raghav@...sung.com,
linux-xfs@...r.kernel.org, catherine.hoang@...cle.com
Subject: Re: [PATCH v3 14/21] iomap: Sub-extent zeroing
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@...cle.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
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