linux-kernel - Re: [PATCH 2/2] [PATCH] bcache: __write_super to handle page sizes other than 4k

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Date:   Mon, 9 Dec 2019 17:52:45 +0800
From:   Coly Li <colyli@...e.de>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     Liang Chen <liangchen.linux@...il.com>, kent.overstreet@...il.com,
        linux-kernel@...r.kernel.org, linux-bcache@...r.kernel.org
Subject: Re: [PATCH 2/2] [PATCH] bcache: __write_super to handle page sizes
 other than 4k

On 2019/12/9 3:37 下午, Christoph Hellwig wrote:
> On Fri, Dec 06, 2019 at 05:44:38PM +0800, Coly Li wrote:
>>>  {
>>> -	struct cache_sb *out = page_address(bio_first_page_all(bio));
>>> +	struct cache_sb *out;
>>>  	unsigned int i;
>>> +	struct buffer_head *bh;
>>> +
>>> +	/*
>>> +	 * The page is held since read_super, this __bread * should not
>>> +	 * cause an extra io read.
>>> +	 */
>>> +	bh = __bread(bdev, 1, SB_SIZE);
>>> +	if (!bh)
>>> +		goto out_bh;
>>> +
>>> +	out = (struct cache_sb *) bh->b_data;
>>
>> This is quite tricky here. Could you please to move this code piece into
>> an inline function and add code comments to explain why a read is
>> necessary for a write.
> 
> A read is not nessecary.  He only added it because he was too fearful
> of calculating the data offset directly.  But calculating it directly
> is almost trivial and should just be done here.  Alternatively if that
> is still to hard just keep a pointer to the cache_sb around, which is
> how most file systems do it.
> 
Copied, if Liang does not have time to handle this as your suggestion, I
will handle it.

Thanks for the hint.

-- 

Coly Li