linux-kernel - Re: [RESEND PATCH v4] fs/ufs: Replace kmap() with kmap_local

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Date:   Fri, 25 Nov 2022 22:50:56 +0000
From:   Al Viro <viro@...iv.linux.org.uk>
To:     "Fabio M. De Francesco" <fmdefrancesco@...il.com>
Cc:     Evgeniy Dushistov <dushistov@...l.ru>,
        Christian Brauner <brauner@...nel.org>,
        linux-kernel@...r.kernel.org, bpf@...r.kernel.org,
        "Venkataramanan, Anirudh" <anirudh.venkataramanan@...el.com>,
        Ira Weiny <ira.weiny@...el.com>
Subject: Re: [RESEND PATCH v4] fs/ufs: Replace kmap() with kmap_local_page()

On Fri, Nov 25, 2022 at 10:17:28PM +0000, Al Viro wrote:

> The bottom line:
> 	* teach your ufs_put_page() to accept any address within the page
> 	* flip the ways you return page and address in ufs_get_page()
> 	* use offset_in_page(addr) instead of these addr - page_address(page)
> and you'll get a much smaller patch, with a lot less noise in it.
> What's more, offset_in_page() part can be carved out into a separate
> commit - it's valid on its own, and it makes both halves easier to
> follow.
> 
> AFAICS, similar observations apply in your sysvfs patch; the point about
> calling conventions for ufs_get_page() definitely applies there, and
> stronger than for ufs - those casts are eye-watering...

As the matter of fact, I'd probably split it into 3 steps:
	1) switch the calling conventions of ufs_get_page() - callers follow
it with something like kaddr = page_address(page); and that change makes
sense on its own.  Isolated and easy to follow.
	2) offset_in_page() changes.  Again, separate, stands on its own and
is easy to follow.
	3) kmap_local() switch itself - pass address to ufs_put_page(), etc.
Considerably smaller and less cluttered than your current variant.