linux-kernel - Re: [PATCH] iomap: use largest_zero_folio() in iomap_dio

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Message-ID: <20250814182713.GS7965@frogsfrogsfrogs>
Date: Thu, 14 Aug 2025 11:27:13 -0700
From: "Darrick J. Wong" <djwong@...nel.org>
To: "Pankaj Raghav (Samsung)" <kernel@...kajraghav.com>
Cc: Christian Brauner <brauner@...nel.org>, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, mcgrof@...nel.org,
	gost.dev@...sung.com, linux-xfs@...r.kernel.org,
	Pankaj Raghav <p.raghav@...sung.com>
Subject: Re: [PATCH] iomap: use largest_zero_folio() in iomap_dio_zero()

On Thu, Aug 14, 2025 at 04:21:37PM +0200, Pankaj Raghav (Samsung) wrote:
> From: Pankaj Raghav <p.raghav@...sung.com>
> 
> iomap_dio_zero() uses a custom allocated memory of zeroes for padding
> zeroes. This was a temporary solution until there was a way to request a
> zero folio that was greater than the PAGE_SIZE.
> 
> Use largest_zero_folio() function instead of using the custom allocated
> memory of zeroes. There is no guarantee from largest_zero_folio()
> function that it will always return a PMD sized folio. Adapt the code so
> that it can also work if largest_zero_folio() returns a ZERO_PAGE.
> 
> Signed-off-by: Pankaj Raghav <p.raghav@...sung.com>

Seems fine to me, though I wonder if this oughn't go along with the
rest of the largest_zero_folio changes?

Reviewed-by: "Darrick J. Wong" <djwong@...nel.org>

--D

> ---
>  fs/iomap/direct-io.c | 38 +++++++++++++++-----------------------
>  1 file changed, 15 insertions(+), 23 deletions(-)
> 
> diff --git a/fs/iomap/direct-io.c b/fs/iomap/direct-io.c
> index b84f6af2eb4c..a7a281ea3e50 100644
> --- a/fs/iomap/direct-io.c
> +++ b/fs/iomap/direct-io.c
> @@ -24,13 +24,6 @@
>  #define IOMAP_DIO_WRITE		(1U << 30)
>  #define IOMAP_DIO_DIRTY		(1U << 31)
>  
> -/*
> - * Used for sub block zeroing in iomap_dio_zero()
> - */
> -#define IOMAP_ZERO_PAGE_SIZE (SZ_64K)
> -#define IOMAP_ZERO_PAGE_ORDER (get_order(IOMAP_ZERO_PAGE_SIZE))
> -static struct page *zero_page;
> -
>  struct iomap_dio {
>  	struct kiocb		*iocb;
>  	const struct iomap_dio_ops *dops;
> @@ -285,24 +278,35 @@ static int iomap_dio_zero(const struct iomap_iter *iter, struct iomap_dio *dio,
>  {
>  	struct inode *inode = file_inode(dio->iocb->ki_filp);
>  	struct bio *bio;
> +	struct folio *zero_folio = largest_zero_folio();
> +	int nr_vecs = max(1, i_blocksize(inode) / folio_size(zero_folio));
>  
>  	if (!len)
>  		return 0;
> +
>  	/*
> -	 * Max block size supported is 64k
> +	 * This limit shall never be reached as most filesystems have a
> +	 * maximum blocksize of 64k.
>  	 */
> -	if (WARN_ON_ONCE(len > IOMAP_ZERO_PAGE_SIZE))
> +	if (WARN_ON_ONCE(nr_vecs > BIO_MAX_VECS))
>  		return -EINVAL;
>  
> -	bio = iomap_dio_alloc_bio(iter, dio, 1, REQ_OP_WRITE | REQ_SYNC | REQ_IDLE);
> +	bio = iomap_dio_alloc_bio(iter, dio, nr_vecs,
> +				  REQ_OP_WRITE | REQ_SYNC | REQ_IDLE);
>  	fscrypt_set_bio_crypt_ctx(bio, inode, pos >> inode->i_blkbits,
>  				  GFP_KERNEL);
>  	bio->bi_iter.bi_sector = iomap_sector(&iter->iomap, pos);
>  	bio->bi_private = dio;
>  	bio->bi_end_io = iomap_dio_bio_end_io;
>  
> -	__bio_add_page(bio, zero_page, len, 0);
> +	while (len > 0) {
> +		unsigned int io_len = min(len, folio_size(zero_folio));
> +
> +		bio_add_folio_nofail(bio, zero_folio, io_len, 0);
> +		len -= io_len;
> +	}
>  	iomap_dio_submit_bio(iter, dio, bio, pos);
> +
>  	return 0;
>  }
>  
> @@ -822,15 +826,3 @@ iomap_dio_rw(struct kiocb *iocb, struct iov_iter *iter,
>  	return iomap_dio_complete(dio);
>  }
>  EXPORT_SYMBOL_GPL(iomap_dio_rw);
> -
> -static int __init iomap_dio_init(void)
> -{
> -	zero_page = alloc_pages(GFP_KERNEL | __GFP_ZERO,
> -				IOMAP_ZERO_PAGE_ORDER);
> -
> -	if (!zero_page)
> -		return -ENOMEM;
> -
> -	return 0;
> -}
> -fs_initcall(iomap_dio_init);
> 
> base-commit: 931e46dcbc7e6035a90e9c4a27a84b660e083f0a
> -- 
> 2.50.1
> 
>