linux-kernel - Re: [PATCH 4/5] arch/kmap_atomic: Consolidate duplicate code

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Message-ID: <20200427011630.GC135929@iweiny-DESK2.sc.intel.com>
Date:   Sun, 26 Apr 2020 18:16:30 -0700
From:   Ira Weiny <ira.weiny@...el.com>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Dan Williams <dan.j.williams@...el.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>,
        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
Subject: Re: [PATCH 4/5] arch/kmap_atomic: Consolidate duplicate code

On Sun, Apr 26, 2020 at 12:26:42AM -0700, Christoph Hellwig wrote:
> > diff --git a/arch/arc/mm/highmem.c b/arch/arc/mm/highmem.c
> > index 4db13a6b9f3b..1cae4b911a33 100644
> > --- a/arch/arc/mm/highmem.c
> > +++ b/arch/arc/mm/highmem.c
> > @@ -53,11 +53,10 @@ void *kmap_atomic(struct page *page)
> >  {
> >  	int idx, cpu_idx;
> >  	unsigned long vaddr;
> > +	void *addr = kmap_atomic_fast(page);
> >  
> > -	preempt_disable();
> > -	pagefault_disable();
> > -	if (!PageHighMem(page))
> > -		return page_address(page);
> > +	if (addr)
> > +		return addr;
> 
> Wouldn't it make sense to just move kmap_atomic itelf to common code,
> and call out to a kmap_atomic_high for the highmem case, following the
> scheme in kmap?
>

Sure I do like that symmetry between the calls.

>
> Same for the unmap side.

FWIW that would simply be renaming  __kunmap_atomic() to kunmap_atomic_high()

>
> That might require to support
> kmap_atomic_prot everywhere first, which sounds like a really good
> idea anyway, and would avoid the need for strange workaround in drm.

Having a kmap_atomic_prot() seems like a good idea.  But I'm not exactly sure
why CONFIG_x86 is being called out specifically in the DRM code?

Ira