linux-kernel - Re: [RFC PATCH v2 7/8] Introduce dcache_is

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Message-ID: <Zbqwx12WfXvZ6kk2@dread.disaster.area>
Date: Thu, 1 Feb 2024 07:42:47 +1100
From: Dave Chinner <david@...morbit.com>
To: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc: Dan Williams <dan.j.williams@...el.com>,
	Vishal Verma <vishal.l.verma@...el.com>,
	Dave Jiang <dave.jiang@...el.com>, linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>, linux-mm@...ck.org,
	linux-arch@...r.kernel.org, Matthew Wilcox <willy@...radead.org>,
	Arnd Bergmann <arnd@...db.de>, Russell King <linux@...linux.org.uk>,
	linux-cxl@...r.kernel.org, nvdimm@...ts.linux.dev,
	linux-xfs@...r.kernel.org
Subject: Re: [RFC PATCH v2 7/8] Introduce dcache_is_aliasing() across all
 architectures

On Wed, Jan 31, 2024 at 09:58:21AM -0500, Mathieu Desnoyers wrote:
> On 2024-01-30 21:48, Dave Chinner wrote:
> > On Tue, Jan 30, 2024 at 11:52:54AM -0500, Mathieu Desnoyers wrote:
> > > Introduce a generic way to query whether the dcache is virtually aliased
> > > on all architectures. Its purpose is to ensure that subsystems which
> > > are incompatible with virtually aliased data caches (e.g. FS_DAX) can
> > > reliably query this.
> > > 
> > > For dcache aliasing, there are three scenarios dependending on the
> > > architecture. Here is a breakdown based on my understanding:
> > > 
> > > A) The dcache is always aliasing:
> > > 
> > > * arc
> > > * csky
> > > * m68k (note: shared memory mappings are incoherent ? SHMLBA is missing there.)
> > > * sh
> > > * parisc
> > 
> > /me wonders why the dentry cache aliasing has problems on these
> > systems.
> > 
> > Oh, dcache != fs/dcache.c (the VFS dentry cache).
> > 
> > Can you please rename this function appropriately so us dumb
> > filesystem people don't confuse cpu data cache configurations with
> > the VFS dentry cache aliasing when we read this code? Something like
> > cpu_dcache_is_aliased(), perhaps?
> 
> Good point, will do. I'm planning go rename as follows for v3 to
> eliminate confusion with dentry cache (and with "page cache" in
> general):
> 
> ARCH_HAS_CACHE_ALIASING -> ARCH_HAS_CPU_CACHE_ALIASING
> dcache_is_aliasing() -> cpu_dcache_is_aliasing()
> 
> I noticed that you suggested "aliased" rather than "aliasing",
> but I followed what arm64 did for icache_is_aliasing(). Do you
> have a strong preference one way or another ?

Not really.

-Dave.
-- 
Dave Chinner
david@...morbit.com