Re: [PATCH V3 (RESEND) 2/3] mm/sparsemem: Enable vmem_altmap support in vmemmap_alloc_block_buf()

From: Anshuman Khandual <anshuman.khandual@arm.com>
To: Catalin Marinas <catalin.marinas@arm.com>
Cc: linux-mm@kvack.org, Will Deacon <will@kernel.org>,
	Benjamin Herrenschmidt <benh@kernel.crashing.org>,
	Paul Mackerras <paulus@samba.org>,
	Michael Ellerman <mpe@ellerman.id.au>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	Andy Lutomirski <luto@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	"H. Peter Anvin" <hpa@zytor.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	x86@kernel.org, linux-arm-kernel@lists.infradead.org,
	linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH V3 (RESEND) 2/3] mm/sparsemem: Enable vmem_altmap support in vmemmap_alloc_block_buf()
Date: Fri, 3 Jul 2020 12:02:11 +0530	[thread overview]
Message-ID: <ef468b16-ca28-7d5d-c5fc-eb2e31de5e03@arm.com> (raw)
In-Reply-To: <20200702140752.GF22241@gaia>

On 07/02/2020 07:37 PM, Catalin Marinas wrote:
> 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.

Okay. Will drop 'fallback' from vmemmap_alloc_block_buf() and update the
callers. There will also be a single change in the subsequent patch i.e
vmemmap_alloc_block_buf(PMD_SIZE, node, altmap).