Re: [PATCH V2 00/11] Subject: Remove duplicated kmap code

From: Al Viro <viro@zeniv.linux.org.uk>
To: ira.weiny@intel.com
Cc: linux-kernel@vger.kernel.org,
	Andrew Morton <akpm@linux-foundation.org>,
	Christian Koenig <christian.koenig@amd.com>,
	Huang Rui <ray.huang@amd.com>,
	Thomas Bogendoerfer <tsbogend@alpha.franken.de>,
	"James E.J. Bottomley" <James.Bottomley@hansenpartnership.com>,
	Helge Deller <deller@gmx.de>,
	Benjamin Herrenschmidt <benh@kernel.crashing.org>,
	Paul Mackerras <paulus@samba.org>,
	"David S. Miller" <davem@davemloft.net>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	Andy Lutomirski <luto@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Chris Zankel <chris@zankel.net>,
	Max Filippov <jcmvbkbc@gmail.com>,
	Dan Williams <dan.j.williams@intel.com>,
	linux-snps-arc@lists.infradead.org,
	linux-arm-kernel@lists.infradead.org, linux-csky@vger.kernel.org,
	linux-mips@vger.kernel.org, linux-parisc@vger.kernel.org,
	linuxppc-dev@lists.ozlabs.org, sparclinux@vger.kernel.org,
	linux-xtensa@linux-xtensa.org, dri-devel@lists.freedesktop.org
Subject: Re: [PATCH V2 00/11] Subject: Remove duplicated kmap code
Date: Mon, 4 May 2020 02:35:09 +0100	[thread overview]
Message-ID: <20200504013509.GU23230@ZenIV.linux.org.uk> (raw)
In-Reply-To: <20200504010912.982044-1-ira.weiny@intel.com>

On Sun, May 03, 2020 at 06:09:01PM -0700, ira.weiny@intel.com wrote:
> From: Ira Weiny <ira.weiny@intel.com>
> 
> The kmap infrastructure has been copied almost verbatim to every architecture.
> This series consolidates obvious duplicated code by defining core functions
> which call into the architectures only when needed.
> 
> Some of the k[un]map_atomic() implementations have some similarities but the
> similarities were not sufficient to warrant further changes.
> 
> In addition we remove a duplicate implementation of kmap() in DRM.
> 
> Testing was done by 0day to cover all the architectures I can't readily
> build/test.

OK...  Looking through my old notes on kmap unification (this winter, never
went anywhere),

* arch/mips/mm/cache.c ought to use linux/highmem.h, not asm/highmem.h
I suspect that your series doesn't build on some configs there.  Hadn't
verified that, though.

* kmap_atomic_to_page() is dead, but not quite gone - csky and nds32 brought
the damn thing back (nds32 - only an extern).  It needs killin'...

* parisc is (arguably) abusing kunmap()/kunmap_atomic() for cache flushing.
Replace the bulk of its highmem.h with
#define ARCH_HAS_FLUSH_ON_KUNMAP
#define arch_before_kunmap flush_kernel_dcache_page_addr
and have default kunmap()/kunmap_atomic() do
#ifdef ARCH_HAS_FLUSH_ON_KUNMAP
	arch_before_kunmap(page_address(page));
#endif
and
#ifdef ARCH_HAS_FLUSH_ON_KUNMAP
	arch_before_kunmap(addr);
#endif
resp.  Kills ARCH_HAS_KMAP along with ifdefs on it, makes parisc use somewhat
less hacky.

I'd suggest checking various configs on mips - that's likely to cause headache.
Said that, my analysis of include chains back then is pretty much worthless
by now - I really hate the amount of indirect include chains leading to that
sucker on some, but not all configs ;-/  IIRC, the proof that everything
using kmap*/kunmap* would pull linux/highmem.h regardless of config took several
hours of digging, ran for several pages and had been hopelessly brittle.
arch/mips/mm/cache.c was the only exception caught by it, but these days
there might be more.