linux-kernel - Re: flush_dcache_page vs kunmap

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Message-ID: <CAHk-=wgY7VOwC9dFGGLZ4_-udzTFkxv6kWbBQg9bSJ5vtHcncA@mail.gmail.com>
Date:   Thu, 4 Nov 2021 14:02:25 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Christoph Hellwig <hch@....de>,
        linux-arch <linux-arch@...r.kernel.org>,
        Ira Weiny <ira.weiny@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: flush_dcache_page vs kunmap_local

On Thu, Nov 4, 2021 at 11:39 AM Matthew Wilcox <willy@...radead.org> wrote:
>
> On Thu, Nov 04, 2021 at 08:30:55AM -0700, Linus Torvalds wrote:
> > Why did this come up? Do you actually have some hardware or situation
> > that cares?
>
> Oh, we're doing review of the XFS/iomap folio patches, which led to
> looking at zero_user_segments(), and I realised that memzero_page()
> was now functionally identical to zero_user().  And you'd been quite
> specific about not having flush_dcache_page() in there, so ... I wondered
> if you'd had a change of mind.

Ugh. I guess it ends up being there whether I like it or not. All that
"zero_user_segments() stuff is too ugly for words, though, so I think
whoever wrote it must have been on some interesting pharmaceuticals.

What the hell are the two start/end things? And most users actually
just want a single page and should never have used that thing. Nasty.

I'm not touching that with a ten-foot pole.

              Linus