linux-kernel - Re: [PATCH v6 05/12] mm: HUGE

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Date:   Sat, 22 Aug 2020 08:45:48 +1000
From:   Nicholas Piggin <npiggin@...il.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Borislav
 Petkov <bp@...en8.de>,
        Catalin Marinas <catalin.marinas@....com>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        Christoph Hellwig <hch@...radead.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Jonathan Cameron <Jonathan.Cameron@...wei.com>,
        linux-arch@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linuxppc-dev@...ts.ozlabs.org, Zefan Li <lizefan@...wei.com>,
        Ingo Molnar <mingo@...hat.com>,
        Thomas
 Gleixner <tglx@...utronix.de>,
        Will Deacon <will@...nel.org>, x86@...nel.org
Subject: Re: [PATCH v6 05/12] mm: HUGE_VMAP arch support cleanup

Excerpts from Andrew Morton's message of August 22, 2020 6:14 am:
> On Sat, 22 Aug 2020 01:12:09 +1000 Nicholas Piggin <npiggin@...il.com> wrote:
> 
>> This changes the awkward approach where architectures provide init
>> functions to determine which levels they can provide large mappings for,
>> to one where the arch is queried for each call.
>> 
>> This removes code and indirection, and allows constant-folding of dead
>> code for unsupported levels.
>> 
>> This also adds a prot argument to the arch query. This is unused
>> currently but could help with some architectures (e.g., some powerpc
>> processors can't map uncacheable memory with large pages).
>> 
>> --- a/arch/arm64/include/asm/vmalloc.h
>> +++ b/arch/arm64/include/asm/vmalloc.h
>> @@ -1,4 +1,12 @@
>>  #ifndef _ASM_ARM64_VMALLOC_H
>>  #define _ASM_ARM64_VMALLOC_H
>>  
>> +#include <asm/page.h>
>> +
>> +#ifdef CONFIG_HAVE_ARCH_HUGE_VMAP
>> +bool arch_vmap_p4d_supported(pgprot_t prot);
>> +bool arch_vmap_pud_supported(pgprot_t prot);
>> +bool arch_vmap_pmd_supported(pgprot_t prot);
>> +#endif
> 
> Moving these out of generic code and into multiple arch headers is
> unfortunate.  Can we leave them in include/linux/somewhere?  And remove
> the ifdefs, if so inclined - they just move the build error from
> link-time to compile-time, and such an error shouldn't occur!

Yeah this was just an intermediate step as you saw. It's a bit 
unfortunate, but I thought it made the arch changes clearer.

Thanks,
Nick