linux-kernel - Re: flush_dcache_page vs kunmap

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Message-ID: <YYQQPuhVUHqfldDg@arm.com>
Date:   Thu, 4 Nov 2021 16:54:22 +0000
From:   Catalin Marinas <catalin.marinas@....com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Matthew Wilcox <willy@...radead.org>,
        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 04, 2021 at 08:30:55AM -0700, Linus Torvalds wrote:
> On Thu, Nov 4, 2021 at 8:03 AM Matthew Wilcox <willy@...radead.org> wrote:
> > Linus offers the opinion that kunmap calls should imply a
> > flush_dcache_page().  Christoph added calls to flush_dcache_page()
> > in commit 8dad53a11f8d.  Was this "voodoo programming", or was there
> > a real problem being addressed?
> 
> I don't think anybody actually uses/cares about flush_dcache_page() at
> all, and pretty much all uses are random and voodoo.

We do. flush_dcache_page() is not just about virtual caches. On arm32/64
(and powerpc), even with PIPT-like caches, we use it to flag a page's
D-cache as no longer clean. Subsequently in set_pte_at(), if the mapping
is executable, we do the cache maintenance to ensure the I and D caches
are coherent with each other.

I wouldn't add this call to kmap/kunmap_local(), it would be a slight
unnecessary overhead (we had a customer complaining about kmap_atomic()
breaking write-streaming, I think the new kmap_local() solved this
problem, if in the right context).

-- 
Catalin