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

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Message-Id: <20191029041013.175636-1-pliard@google.com>
Date:   Tue, 29 Oct 2019 13:10:13 +0900
From:   Philippe Liard <pliard@...gle.com>
To:     phillip@...ashfs.org.uk, hch@....de
Cc:     linux-kernel@...r.kernel.org, groeck@...omium.org,
        pliard@...gle.com
Subject: Re: [PATCH] squashfs: Migrate from ll_rw_block usage to BIO

> > I don't see why you still need buffer_heads at all.  Basically if
> > you replace each of your allocated buffer heads with a simple page
> > allocation the code will be much simpler (this version adds more
> > than 100 lines of code!) and probaby still a bit faster as you
> > don't need the squashfs_bio_request allocation either.
>
> Thanks Christoph 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.

Christoph, do you still think this could be simplified as you
suggested?