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Date:   Wed, 21 Jun 2023 11:05:57 +0200
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Subject: Re: [RFC 3/4] block: set mapping order for the block cache in
 set_init_blocksize
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To:     Pankaj Raghav <p.raghav@samsung.com>, willy@infradead.org,
        david@fromorbit.com
Cc:     gost.dev@samsung.com, mcgrof@kernel.org, hch@lst.de,
        jwong@kernel.org, linux-fsdevel@vger.kernel.org,
        linux-kernel@vger.kernel.org
References: <20230621083823.1724337-1-p.raghav@samsung.com>
 <CGME20230621083828eucas1p23222cae535297f9536f12dddd485f97b@eucas1p2.samsung.com>
 <20230621083823.1724337-4-p.raghav@samsung.com>
From:   Hannes Reinecke <hare@suse.de>
In-Reply-To: <20230621083823.1724337-4-p.raghav@samsung.com>
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On 6/21/23 10:38, Pankaj Raghav wrote:
> From: Luis Chamberlain <mcgrof@kernel.org>
> 
> Automatically set the minimum mapping order for block devices in
> set_init_blocksize(). The mapping order will be set only when the block
> device uses iomap based aops.
> 
> Signed-off-by: Pankaj Raghav <p.raghav@samsung.com>
> Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>
> ---
>   block/bdev.c | 9 +++++++++
>   1 file changed, 9 insertions(+)
> 
> diff --git a/block/bdev.c b/block/bdev.c
> index 9bb54d9d02a6..db8cede8a320 100644
> --- a/block/bdev.c
> +++ b/block/bdev.c
> @@ -126,6 +126,7 @@ static void set_init_blocksize(struct block_device *bdev)
>   {
>   	unsigned int bsize = bdev_logical_block_size(bdev);
>   	loff_t size = i_size_read(bdev->bd_inode);
> +	int order, folio_order;
>   
>   	while (bsize < PAGE_SIZE) {
>   		if (size & bsize)
> @@ -133,6 +134,14 @@ static void set_init_blocksize(struct block_device *bdev)
>   		bsize <<= 1;
>   	}
>   	bdev->bd_inode->i_blkbits = blksize_bits(bsize);
> +	order = bdev->bd_inode->i_blkbits - PAGE_SHIFT;
> +	folio_order = mapping_min_folio_order(bdev->bd_inode->i_mapping);
> +
> +	if (!IS_ENABLED(CONFIG_BUFFER_HEAD)) {
> +		/* Do not allow changing the folio order after it is set */
> +		WARN_ON_ONCE(folio_order && (folio_order != order));
> +		mapping_set_folio_orders(bdev->bd_inode->i_mapping, order, 31);
> +	}
>   }
>   
>   int set_blocksize(struct block_device *bdev, int size)
This really has nothing to do with buffer heads.

In fact, I've got a patchset to make it work _with_ buffer heads.

So please, don't make it conditional on CONFIG_BUFFER_HEAD.

And we should be calling into 'mapping_set_folio_order()' only if the 
'order' argument is larger than PAGE_ORDER, otherwise we end up enabling
large folio support for _every_ block device.
Which I doubt we want.

Cheers,

Hannes