linux-kernel - Re: [PATCH 2/7] block: align the bio after building it

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Message-ID: <be0733c0-0ca7-4359-a979-7cc55ec24fa6@suse.de>
Date: Tue, 5 Aug 2025 08:54:58 +0200
From: Hannes Reinecke <hare@...e.de>
To: Keith Busch <kbusch@...nel.org>
Cc: Keith Busch <kbusch@...a.com>, linux-block@...r.kernel.org,
 linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
 snitzer@...nel.org, axboe@...nel.dk, dw@...idwei.uk, brauner@...nel.org
Subject: Re: [PATCH 2/7] block: align the bio after building it

On 8/4/25 16:08, Keith Busch wrote:
> On Mon, Aug 04, 2025 at 08:54:00AM +0200, Hannes Reinecke wrote:
>> On 8/2/25 01:47, Keith Busch wrote:
>>> +static int bio_align_to_lbs(struct bio *bio, struct iov_iter *iter)
>>> +{
>>> +	struct block_device *bdev = bio->bi_bdev;
>>> +	size_t nbytes;
>>> +
>>> +	if (!bdev)
>>> +		return 0;
>>> +
>>> +	nbytes = bio->bi_iter.bi_size & (bdev_logical_block_size(bdev) - 1);
>>> +	if (!nbytes)
>>> +		return 0;
>>> +
>>> +	bio_revert(bio, nbytes);
>>> +	iov_iter_revert(iter, nbytes);
>>> +	if (!bio->bi_iter.bi_size)
>>> +		return -EFAULT;
>>> +
>>> +	return 0;
>>> +}
>>> +
>>>    /**
>>>     * bio_iov_iter_get_pages - add user or kernel pages to a bio
>>>     * @bio: bio to add pages to
>>> @@ -1336,6 +1355,7 @@ int bio_iov_iter_get_pages(struct bio *bio, struct iov_iter *iter)
>>>    		ret = __bio_iov_iter_get_pages(bio, iter);
>>>    	} while (!ret && iov_iter_count(iter) && !bio_full(bio, 0));
>>> +	ret = bio_align_to_lbs(bio, iter);
>>>    	return bio->bi_vcnt ? 0 : ret;
>>
>> Wouldn't that cause the error from bio_align_to_lba() to be ignored
>> if bio->bi_vcnt is greater than 0?
> 
> That returns an error only if the alignment reduces the size to 0, so
> there would be a bug somewhere if bi_vcnt is not also 0 in that case.

It would, but we wouldn't be seeing it as 'ret' would be obscured
if 'bio->bi_vcnt' continues to be greater than zero and 'ret' is set.

Cheers,

Hannes
-- 
Dr. Hannes Reinecke                  Kernel Storage Architect
hare@...e.de                                +49 911 74053 688
SUSE Software Solutions GmbH, Frankenstr. 146, 90461 Nürnberg
HRB 36809 (AG Nürnberg), GF: I. Totev, A. McDonald, W. Knoblich