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Date:   Wed, 21 Jun 2023 13:02:32 +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>
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 <d275b49a-b6be-a08f-cfd8-d213eb452dd1@samsung.com>
From:   Hannes Reinecke <hare@suse.de>
In-Reply-To: <d275b49a-b6be-a08f-cfd8-d213eb452dd1@samsung.com>
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On 6/21/23 12:42, Pankaj Raghav wrote:
>>>        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.
>>
> 
> Hmm, which aops are you using for the block device? If you are using the old aops, then we will be
> using helpers from buffer.c and mpage.c which do not support large folios. I am getting a BUG_ON
> when I don't use iomap based aops for the block device:
> 
I know. I haven't said that mpage.c / buffer.c support large folios 
_now_. All I'm saying is that I have a patchset enabling it to support 
large folios :-)

Cheers,

Hannes