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

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Date:   Fri, 21 Aug 2020 13:14:22 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Nicholas Piggin <npiggin@...il.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        linux-arch@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        Zefan Li <lizefan@...wei.com>,
        Jonathan Cameron <Jonathan.Cameron@...wei.com>,
        Christoph Hellwig <hch@...radead.org>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        linux-arm-kernel@...ts.infradead.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH v6 05/12] mm: HUGE_VMAP arch support cleanup

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!