linux-kernel - Re: [PATCH v3 17/21] iomap: Atomic write support

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Message-ID: <ZjGfOyJl5y3D49fC@dread.disaster.area>
Date: Wed, 1 May 2024 11:47:39 +1000
From: Dave Chinner <david@...morbit.com>
To: John Garry <john.g.garry@...cle.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 17/21] iomap: Atomic write support

On Mon, Apr 29, 2024 at 05:47:42PM +0000, John Garry wrote:
> Support atomic writes by producing a single BIO with REQ_ATOMIC flag set.
> 
> We rely on the FS to guarantee extent alignment, such that an atomic write
> should never straddle two or more extents. The FS should also check for
> validity of an atomic write length/alignment.
> 
> Signed-off-by: John Garry <john.g.garry@...cle.com>
> ---
>  fs/iomap/direct-io.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> diff --git a/fs/iomap/direct-io.c b/fs/iomap/direct-io.c
> index a3ed7cfa95bc..d7bdeb675068 100644
> --- a/fs/iomap/direct-io.c
> +++ b/fs/iomap/direct-io.c
> @@ -275,6 +275,7 @@ static inline blk_opf_t iomap_dio_bio_opflags(struct iomap_dio *dio,
>  static loff_t iomap_dio_bio_iter(const struct iomap_iter *iter,
>  		struct iomap_dio *dio)
>  {
> +	bool is_atomic = dio->iocb->ki_flags & IOCB_ATOMIC;
>  	const struct iomap *iomap = &iter->iomap;
>  	struct inode *inode = iter->inode;
>  	unsigned int zeroing_size, pad;
> @@ -387,6 +388,9 @@ static loff_t iomap_dio_bio_iter(const struct iomap_iter *iter,
>  		bio->bi_iter.bi_sector = iomap_sector(iomap, pos);
>  		bio->bi_write_hint = inode->i_write_hint;
>  		bio->bi_ioprio = dio->iocb->ki_ioprio;
> +		if (is_atomic)
> +			bio->bi_opf |= REQ_ATOMIC;

REQ_ATOMIC is only valid for write IO, isn't it?

This should be added in iomap_dio_bio_opflags() after it is
determined we are doing a write operation.  Regardless, it should be
added in iomap_dio_bio_opflags(), not here. That also allows us to
get rid of the is_atomic variable.

> +
>  		bio->bi_private = dio;
>  		bio->bi_end_io = iomap_dio_bio_end_io;
>  
> @@ -403,6 +407,12 @@ static loff_t iomap_dio_bio_iter(const struct iomap_iter *iter,
>  		}
>  
>  		n = bio->bi_iter.bi_size;
> +		if (is_atomic && n != orig_count) {
> +			/* This bio should have covered the complete length */
> +			ret = -EINVAL;
> +			bio_put(bio);
> +			goto out;
> +		}

What happens now if we've done zeroing IO before this? I suspect we
might expose stale data if the partial block zeroing converts the
unwritten extent in full...

>  		if (dio->flags & IOMAP_DIO_WRITE) {
>  			task_io_account_write(n);
>  		} else {

Ignoring the error handling issues, this code might be better as:

		if (dio->flags & IOMAP_DIO_WRITE) {
			if ((opflags & REQ_ATOMIC) && n != orig_count) {
				/* atomic writes are all or nothing */
				ret = -EIO
				bio_put(bio);
				goto out;
			}
		}

so that we are not putting atomic write error checks in the read IO
submission path.

-Dave.
-- 
Dave Chinner
david@...morbit.com