linux-kernel - Re: [PATCH V1 05/10] arch/kmap_atomic: Consolidate duplicate code

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Date:   Fri, 1 May 2020 03:09:12 +0100
From:   Al Viro <viro@...iv.linux.org.uk>
To:     ira.weiny@...el.com
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 V1 05/10] arch/kmap_atomic: Consolidate duplicate code

On Thu, Apr 30, 2020 at 01:38:40PM -0700, ira.weiny@...el.com wrote:
> From: Ira Weiny <ira.weiny@...el.com>
> 
> Every arch has the same code to ensure atomic operations and a check for
> !HIGHMEM page.
> 
> Remove the duplicate code by defining a core kmap_atomic() which only
> calls the arch specific kmap_atomic_high() when the page is high memory.

Err....  AFAICS, you've just silently changed the semantics for
kmap_atomic_prot() here.  And while most of the callers are converted,
drivers/gpu/drm/ttm/ttm_bo_util.c one is not, so at the very least it's
a bisect hazard...

And I would argue that having kmap_atomic() differ from kmap_atomic_prot()
wrt disabling preempt is asking for trouble.