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

[Christoph Hellwig]

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.