linux-kernel - Re: [PATCH] squashfs: Migrate from ll_rw

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Message-Id: <20191024012354.105261-1-pliard@google.com>
Date:   Thu, 24 Oct 2019 10:23:54 +0900
From:   Philippe Liard <pliard@...gle.com>
To:     phillip@...ashfs.org.uk
Cc:     linux-kernel@...r.kernel.org, groeck@...omium.org,
        pliard@...gle.com
Subject: Re: [PATCH] squashfs: Migrate from ll_rw_block usage to BIO

Thanks Cristoph for taking a look. I like the idea of simplifying this if
possible. I think I understand your suggestion in principle but I'm not
seeing a way to apply it here. Would it be possible for you to be a little
more specific? Let me try to explain this below.

My admittedly limited understanding is that using BIO indirectly requires
buffer_head or an alternative including some synchronization mechanism at
least.
It's true that the bio_{alloc,add_page,submit}() functions don't require
passing a buffer_head. However because bio_submit() is asynchronous AFAICT
the client needs to use a synchronization mechanism to wait for and notify
the completion of the request which buffer heads provide. This is achieved
respectively by wait_on_buffer() and {set,clear}_buffer_uptodate().

Another dependency on buffer heads is the fact that squashfs_read_data()
calls into other squashfs functions operating on buffer heads outside this
file. For example squashfs_decompress() operates on a buffer_head array.

Given that bio_submit() is asynchronous I'm also not seeing how the
squashfs_bio_request allocation can be removed? There can be multiple BIO
requests in flight each needing to carry some context used on completion
of the request.