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

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Date:   Fri, 6 Dec 2019 01:23:36 -0800
From:   Christoph Hellwig <hch@...radead.org>
To:     Liang Chen <liangchen.linux@...il.com>
Cc:     colyli@...e.de, 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 Fri, Dec 06, 2019 at 04:55:43PM +0800, Liang Chen wrote:
> __write_super assumes super block data starts at offset 0 of the page
> read in with __bread from read_super, which is not true when page size
> is not 4k. We encountered the issue on system with 64K page size - commonly
>  seen on aarch64 architecture.
> 
> Instead of making any assumption on the offset of the data within the page,
> this patch calls __bread again to locate the data. That should not introduce
> an extra io since the page has been held when it's read in from read_super,
> and __write_super is not on performance critical code path.

No need to use buffer heads here, you can just use offset_in_page
to calculate the offset.  Similarly I think the read side shouldn't
use buffer heads either (it is the only use of buffer heads in bcache!),
a siple read_cache_page should be all that is needed.