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

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Message-ID: <14c5a629-2169-4271-97b8-a1aba45a6e54@suse.de>
Date: Mon, 4 Aug 2025 08:54:00 +0200
From: Hannes Reinecke <hare@...e.de>
To: Keith Busch <kbusch@...a.com>, linux-block@...r.kernel.org,
 linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Cc: snitzer@...nel.org, axboe@...nel.dk, dw@...idwei.uk, brauner@...nel.org,
 Keith Busch <kbusch@...nel.org>
Subject: Re: [PATCH 2/7] block: align the bio after building it

On 8/2/25 01:47, Keith Busch wrote:
> From: Keith Busch <kbusch@...nel.org>
> 
> Ensure the entire size is aligned after it's built instead of ensuring
> each vector is block size aligned while constructing it. This makes it
> more flexible to accepting device valid vectors that would otherwise get
> rejected by overzealous alignment checks.
> 
> Signed-off-by: Keith Busch <kbusch@...nel.org>
> ---
>   block/bio.c | 58 +++++++++++++++++++++++++++++++++++------------------
>   1 file changed, 39 insertions(+), 19 deletions(-)
> 
> diff --git a/block/bio.c b/block/bio.c
> index 92c512e876c8d..c050903e1be0c 100644
> --- a/block/bio.c
> +++ b/block/bio.c
> @@ -1227,13 +1227,6 @@ static int __bio_iov_iter_get_pages(struct bio *bio, struct iov_iter *iter)
>   	if (bio->bi_bdev && blk_queue_pci_p2pdma(bio->bi_bdev->bd_disk->queue))
>   		extraction_flags |= ITER_ALLOW_P2PDMA;
>   
> -	/*
> -	 * Each segment in the iov is required to be a block size multiple.
> -	 * However, we may not be able to get the entire segment if it spans
> -	 * more pages than bi_max_vecs allows, so we have to ALIGN_DOWN the
> -	 * result to ensure the bio's total size is correct. The remainder of
> -	 * the iov data will be picked up in the next bio iteration.
> -	 */
>   	size = iov_iter_extract_pages(iter, &pages,
>   				      UINT_MAX - bio->bi_iter.bi_size,
>   				      nr_pages, extraction_flags, &offset);
> @@ -1241,18 +1234,6 @@ static int __bio_iov_iter_get_pages(struct bio *bio, struct iov_iter *iter)
>   		return size ? size : -EFAULT;
>   
>   	nr_pages = DIV_ROUND_UP(offset + size, PAGE_SIZE);
> -
> -	if (bio->bi_bdev) {
> -		size_t trim = size & (bdev_logical_block_size(bio->bi_bdev) - 1);
> -		iov_iter_revert(iter, trim);
> -		size -= trim;
> -	}
> -
> -	if (unlikely(!size)) {
> -		ret = -EFAULT;
> -		goto out;
> -	}
> -
>   	for (left = size, i = 0; left > 0; left -= len, i += num_pages) {
>   		struct page *page = pages[i];
>   		struct folio *folio = page_folio(page);
> @@ -1297,6 +1278,44 @@ static int __bio_iov_iter_get_pages(struct bio *bio, struct iov_iter *iter)
>   	return ret;
>   }
>   
> +static inline void bio_revert(struct bio *bio, unsigned int nbytes)
> +{
> +	bio->bi_iter.bi_size -= nbytes;
> +
> +	while (nbytes) {
> +		struct bio_vec *bv = &bio->bi_io_vec[bio->bi_vcnt - 1];
> +
> +		if (nbytes < bv->bv_len) {
> +			bv->bv_len -= nbytes;
> +			return;
> +		}
> +
> +		bio_release_page(bio, bv->bv_page);
> +		bio->bi_vcnt--;
> +		nbytes -= bv->bv_len;
> +       }
> +}
> +
> +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?

>   }
>   EXPORT_SYMBOL_GPL(bio_iov_iter_get_pages);

Cheers,

Hsnnes
-- 
Dr. Hannes Reinecke                  Kernel Storage Architect
hare@...e.de                                +49 911 74053 688
SUSE Software Solutions GmbH, Frankenstr. 146, 90461 Nürnberg
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