linux-kernel - Re: [PATCH V4 1/3] mm/sparsemem: Enable vmem_altmap support in vmemmap_populate

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Message-ID: <eeeb44f9-bc0a-2c3d-8e8f-7e3d9e066c7e@redhat.com>
Date:   Mon, 6 Jul 2020 11:03:47 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Anshuman Khandual <anshuman.khandual@....com>, linux-mm@...ck.org
Cc:     justin.he@....com, catalin.marinas@....com,
        akpm@...ux-foundation.org, Will Deacon <will@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Tony Luck <tony.luck@...el.com>,
        Fenghua Yu <fenghua.yu@...el.com>,
        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>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Michal Hocko <mhocko@...e.com>,
        "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Pavel Tatashin <pasha.tatashin@...een.com>,
        linux-arm-kernel@...ts.infradead.org, linux-ia64@...r.kernel.org,
        linux-riscv@...ts.infradead.org, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH V4 1/3] mm/sparsemem: Enable vmem_altmap support in
 vmemmap_populate_basepages()

>  	return 0;
> @@ -1505,7 +1505,7 @@ int __meminit vmemmap_populate(unsigned long start, unsigned long end, int node,
>  	int err;
>  
>  	if (end - start < PAGES_PER_SECTION * sizeof(struct page))
> -		err = vmemmap_populate_basepages(start, end, node);
> +		err = vmemmap_populate_basepages(start, end, node, NULL);
>  	else if (boot_cpu_has(X86_FEATURE_PSE))
>  		err = vmemmap_populate_hugepages(start, end, node, altmap);
>  	else if (altmap) {

It's somewhat weird that we don't allocate basepages from altmap on x86
(both for sub-sections and without PSE). I wonder if we can simply
unlock that with your change. Especially, also handle the
!X86_FEATURE_PSE case below properly with an altmap.

a) all hw with PMEM has PSE - except special QEMU setups, so nobody
cared to implement. For the sub-section special case, nobody cared about
a handfull of memmap not ending up on the altmap. (but it's still wasted
system memory IIRC).

b) the pagetable overhead for small pages is not-neglectable and might
result in similar issues as solved by the switch to altmap on very huge
PMEM (with small amount of system RAM).

I guess it is due to a).

[...]

>  
> -pte_t * __meminit vmemmap_pte_populate(pmd_t *pmd, unsigned long addr, int node)
> +pte_t * __meminit vmemmap_pte_populate(pmd_t *pmd, unsigned long addr, int node,
> +				       struct vmem_altmap *altmap)
>  {
>  	pte_t *pte = pte_offset_kernel(pmd, addr);
>  	if (pte_none(*pte)) {
>  		pte_t entry;
> -		void *p = vmemmap_alloc_block_buf(PAGE_SIZE, node);
> +		void *p;
> +
> +		if (altmap)
> +			p = altmap_alloc_block_buf(PAGE_SIZE, altmap);
> +		else
> +			p = vmemmap_alloc_block_buf(PAGE_SIZE, node);
>  		if (!p)
>  			return NULL;

I was wondering if

if (altmap)
	p = altmap_alloc_block_buf(PAGE_SIZE, altmap);
if (!p)
	p = vmemmap_alloc_block_buf(PAGE_SIZE, node);
if (!p)
	return NULL

Would make sense. But I guess this isn't really relevant in practice,
because the altmap is usually sized properly.

In general, LGTM.

-- 
Thanks,

David / dhildenb