linux-kernel - Re: [PATCH V3 (RESEND) 2/3] mm/sparsemem: Enable vmem_altmap support in vmemmap_alloc_block

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Date:   Thu, 2 Jul 2020 15:07:52 +0100
From:   Catalin Marinas <catalin.marinas@....com>
To:     Anshuman Khandual <anshuman.khandual@....com>
Cc:     linux-mm@...ck.org, Will Deacon <will@...nel.org>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Andrew Morton <akpm@...ux-foundation.org>, x86@...nel.org,
        linux-arm-kernel@...ts.infradead.org,
        linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH V3 (RESEND) 2/3] mm/sparsemem: Enable vmem_altmap support
 in vmemmap_alloc_block_buf()

On Thu, Jun 18, 2020 at 06:45:29AM +0530, Anshuman Khandual wrote:
> There are many instances where vmemap allocation is often switched between
> regular memory and device memory just based on whether altmap is available
> or not. vmemmap_alloc_block_buf() is used in various platforms to allocate
> vmemmap mappings. Lets also enable it to handle altmap based device memory
> allocation along with existing regular memory allocations. This will help
> in avoiding the altmap based allocation switch in many places.
> 
> While here also implement a regular memory allocation fallback mechanism
> when the first preferred device memory allocation fails. This will ensure
> preserving the existing semantics on powerpc platform. To summarize there
> are three different methods to call vmemmap_alloc_block_buf().
> 
> (., NULL,   false) /* Allocate from system RAM */
> (., altmap, false) /* Allocate from altmap without any fallback */
> (., altmap, true)  /* Allocate from altmap with fallback (system RAM) */
[...]
> diff --git a/arch/powerpc/mm/init_64.c b/arch/powerpc/mm/init_64.c
> index bc73abf0bc25..01e25b56eccb 100644
> --- a/arch/powerpc/mm/init_64.c
> +++ b/arch/powerpc/mm/init_64.c
> @@ -225,12 +225,12 @@ int __meminit vmemmap_populate(unsigned long start, unsigned long end, int node,
>  		 * fall back to system memory if the altmap allocation fail.
>  		 */
>  		if (altmap && !altmap_cross_boundary(altmap, start, page_size)) {
> -			p = altmap_alloc_block_buf(page_size, altmap);
> -			if (!p)
> -				pr_debug("altmap block allocation failed, falling back to system memory");
> +			p = vmemmap_alloc_block_buf(page_size, node,
> +						    altmap, true);
> +		} else {
> +			p = vmemmap_alloc_block_buf(page_size, node,
> +						    NULL, false);
>  		}
> -		if (!p)
> -			p = vmemmap_alloc_block_buf(page_size, node);
>  		if (!p)
>  			return -ENOMEM;

Is the fallback argument actually necessary. It may be cleaner to just
leave the code as is with the choice between altmap and NULL. If an arch
needs a fallback (only powerpc), they have the fallback in place
already. I don't see the powerpc code any better after this change.

I'm fine with the altmap argument though.

-- 
Catalin