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

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Date:   Sun, 3 May 2020 22:04:47 -0700
From:   Ira Weiny <ira.weiny@...el.com>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Christian Koenig <christian.koenig@....com>,
        Huang Rui <ray.huang@....com>,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        "James E.J. Bottomley" <James.Bottomley@...senpartnership.com>,
        Helge Deller <deller@....de>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        "David S. Miller" <davem@...emloft.net>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Chris Zankel <chris@...kel.net>,
        Max Filippov <jcmvbkbc@...il.com>,
        Dan Williams <dan.j.williams@...el.com>,
        linux-snps-arc@...ts.infradead.org,
        linux-arm-kernel@...ts.infradead.org, linux-csky@...r.kernel.org,
        linux-mips@...r.kernel.org, linux-parisc@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, sparclinux@...r.kernel.org,
        linux-xtensa@...ux-xtensa.org, dri-devel@...ts.freedesktop.org
Subject: Re: [PATCH V2 00/11] Subject: Remove duplicated kmap code

On Mon, May 04, 2020 at 02:35:09AM +0100, Al Viro wrote:
> On Sun, May 03, 2020 at 06:09:01PM -0700, ira.weiny@...el.com wrote:
> > From: Ira Weiny <ira.weiny@...el.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.

Yes patch 6 makes the change because kmap_atomic() was no longer declared in
asm/highmem.h.  I'm pretty sure 0-day caught that ...  but I seem to remember
noticing some oddness in that file and I did go through it by hand.

> 
> * 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'...

Easy enough. Added as a follow on patch.

> 
> * 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.

Agreed.  Done in a follow on patch.

> 
> 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.

Grepping for 'asm/highmem.h' and investigations don't reveal any issues...  But
you do have me worried.  That said 0-day has been crunching on multiple
versions of this series without issues such as this (save the mips issue
above).

I have to say it would be nice if the relation between linux/highmem.h and
asm/highmem.h was more straightforward.

Ira