Re: [PATCH v3 10/15] block: Add fops atomic write support

[John Garry]

On 13/02/2024 09:36, Nilay Shroff wrote:
> > +static bool blkdev_atomic_write_valid(struct block_device *bdev, loff_t pos,
>
> > +				      struct iov_iter *iter)
>
> > +{
>
> > +	struct request_queue *q = bdev_get_queue(bdev);
>
> > +	unsigned int min_bytes = queue_atomic_write_unit_min_bytes(q);
>
> > +	unsigned int max_bytes = queue_atomic_write_unit_max_bytes(q);
>
> > +
>
> > +	if (!iter_is_ubuf(iter))
>
> > +		return false;
>
> > +	if (iov_iter_count(iter) & (min_bytes - 1))
>
> > +		return false;
>
> > +	if (!is_power_of_2(iov_iter_count(iter)))
>
> > +		return false;
>
> > +	if (pos & (iov_iter_count(iter) - 1))
>
> > +		return false;
>
> > +	if (iov_iter_count(iter) > max_bytes)
>
> > +		return false;
>
> > +	return true;
>
> > +}
>
>
>
> Here do we need to also validate whether the IO doesn't straddle
>
> the atmic bondary limit (if it's non-zero)? We do check that IO
>
> doesn't straddle the atomic boundary limit but that happens very
>
> late in the IO code path either during blk-merge or in NVMe driver
>
> code.

It's relied that atomic_write_unit_max is <= atomic_write_boundary and 
both are a power-of-2. Please see the NVMe patch, which this is checked. 
Indeed, it would not make sense if atomic_write_unit_max > 
atomic_write_boundary (when non-zero).

So if the write is naturally aligned and its size is <= 
atomic_write_unit_max, then it cannot be straddling a boundary.

Thanks,
John