linux-kernel - Re: [PATCH V1 09/10] arch/kmap: Define kmap_atomic

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Message-ID: <20200501032020.GG23230@ZenIV.linux.org.uk>
Date:   Fri, 1 May 2020 04:20:20 +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 09/10] arch/kmap: Define kmap_atomic_prot() for all
 arch's

On Fri, May 01, 2020 at 03:37:34AM +0100, Al Viro wrote:
> On Thu, Apr 30, 2020 at 01:38:44PM -0700, ira.weiny@...el.com wrote:
> 
> > -static inline void *kmap_atomic(struct page *page)
> > +static inline void *kmap_atomic_prot(struct page *page, pgprot_t prot)
> >  {
> >  	preempt_disable();
> >  	pagefault_disable();
> >  	if (!PageHighMem(page))
> >  		return page_address(page);
> > -	return kmap_atomic_high(page);
> > +	return kmap_atomic_high_prot(page, prot);
> >  }
> > +#define kmap_atomic(page)	kmap_atomic_prot(page, kmap_prot)
> 
> OK, so it *was* just a bisect hazard - you return to original semantics
> wrt preempt_disable()...

FWIW, how about doing the following: just before #5/10 have a patch
that would touch only microblaze, ppc and x86 splitting their
kmap_atomic_prot() into an inline helper + kmap_atomic_high_prot().
Then your #5 would leave their kmap_atomic_prot() as-is (it would
use kmap_atomic_prot_high() instead).  The rest of the series plays
out pretty much the same way it does now, and wrappers on those
3 architectures would go away when an identical generic one is
introduced in this commit (#9/10).

AFAICS, that would avoid the bisect hazard and might even end
up with less noise in the patches...