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

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Date: Tue, 13 Feb 2024 09:58:52 +0000
From: John Garry <john.g.garry@...cle.com>
To: Nilay Shroff <nilay@...ux.ibm.com>
Cc: axboe@...nel.dk, brauner@...nel.org, bvanassche@....org,
        dchinner@...hat.com, djwong@...nel.org, hch@....de, jack@...e.cz,
        jbongio@...gle.com, jejb@...ux.ibm.com, kbusch@...nel.org,
        linux-block@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-nvme@...ts.infradead.org,
        linux-scsi@...r.kernel.org, linux-xfs@...r.kernel.org,
        martin.petersen@...cle.com, ming.lei@...hat.com, ojaswin@...ux.ibm.com,
        sagi@...mberg.me, tytso@....edu, viro@...iv.linux.org.uk
Subject: Re: [PATCH v3 10/15] block: Add fops atomic write support

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