Subject: Re: [PATCH v3 8/8] mm/vmalloc: Hugepage vmalloc mappings

From: Jonathan Cameron <[email-masked]>
Date: Wed, 12 Aug 2020 17:18:33 +0100

On Wed, 12 Aug 2020 13:25:24 +0100
Jonathan Cameron <[email-masked]> wrote:

> On Mon, 10 Aug 2020 12:27:32 +1000
> Nicholas Piggin <[email-masked]> wrote:
> 
> > On platforms that define HAVE_ARCH_HUGE_VMAP and support PMD vmaps,
> > vmalloc will attempt to allocate PMD-sized pages first, before falling
> > back to small pages.
> > 
> > Allocations which use something other than PAGE_KERNEL protections are
> > not permitted to use huge pages yet, not all callers expect this (e.g.,
> > module allocations vs strict module rwx).
> > 
> > This reduces TLB misses by nearly 30x on a `git diff` workload on a
> > 2-node POWER9 (59,800 -> 2,100) and reduces CPU cycles by 0.54%.
> > 
> > This can result in more internal fragmentation and memory overhead for a
> > given allocation, an option nohugevmap is added to disable at boot.
> > 
> > Signed-off-by: Nicholas Piggin <[email-masked]>  
> Hi Nicholas,
> 
> Busy afternoon, but a possible point of interest in line in the meantime.
> 

I did manage to get back to this.

The issue I think is that ARM64 defines THREAD_ALIGN with CONFIG_VMAP_STACK
to be 2* THREAD SIZE.  There is comment in arch/arm64/include/asm/memory.h
that this is to allow cheap checking of overflow.

A quick grep suggests ARM64 is the only architecture to do this...

Jonathan

> 
> ...
> 
> > @@ -2701,22 +2760,45 @@ void *__vmalloc_node_range(unsigned long size, unsigned long align,
> >  			pgprot_t prot, unsigned long vm_flags, int node,
> >  			const void *caller)
> >  {
> > -	struct vm_struct *area;
> > +	struct vm_struct *area = NULL;
> >  	void *addr;
> >  	unsigned long real_size = size;
> > +	unsigned long real_align = align;
> > +	unsigned int shift = PAGE_SHIFT;
> >  
> >  	size = PAGE_ALIGN(size);
> >  	if (!size || (size >> PAGE_SHIFT) > totalram_pages())
> >  		goto fail;
> >  
> > -	area = __get_vm_area_node(real_size, align, VM_ALLOC | VM_UNINITIALIZED |
> > +	if (vmap_allow_huge && (pgprot_val(prot) == pgprot_val(PAGE_KERNEL))) {
> > +		unsigned long size_per_node;
> > +
> > +		/*
> > +		 * Try huge pages. Only try for PAGE_KERNEL allocations,
> > +		 * others like modules don't yet expect huge pages in
> > +		 * their allocations due to apply_to_page_range not
> > +		 * supporting them.
> > +		 */
> > +
> > +		size_per_node = size;
> > +		if (node == NUMA_NO_NODE)
> > +			size_per_node /= num_online_nodes();
> > +		if (size_per_node >= PMD_SIZE)
> > +			shift = PMD_SHIFT;
> > +	}
> > +
> > +again:
> > +	align = max(real_align, 1UL << shift);
> > +	size = ALIGN(real_size, align);  
> 
> So my suspicion is that the issue on arm64 is related to this.
> In the relevant call path, align is 32K whilst the size is 16K
> 
> Previously I don't think we force size to be a multiple of align.
> 
> I think this results in nr_pages being double what it was before.
> 
> 
> > +
> > +	area = __get_vm_area_node(size, align, VM_ALLOC | VM_UNINITIALIZED |
> >  				vm_flags, start, end, node, gfp_mask, caller);
> >  	if (!area)
> >  		goto fail;
> >  
> > -	addr = __vmalloc_area_node(area, gfp_mask, prot, node);
> > +	addr = __vmalloc_area_node(area, gfp_mask, prot, shift, node);
> >  	if (!addr)
> > -		return NULL;
> > +		goto fail;
> >  
> >  	/*
> >  	 * In this function, newly allocated vm_struct has VM_UNINITIALIZED
> > @@ -2730,8 +2812,16 @@ void *__vmalloc_node_range(unsigned long size, unsigned long align,
> >  	return addr;
> >  
> >  fail:
> > -	warn_alloc(gfp_mask, NULL,
> > +	if (shift > PAGE_SHIFT) {
> > +		shift = PAGE_SHIFT;
> > +		goto again;
> > +	}
> > +
> > +	if (!area) {
> > +		/* Warn for area allocation, page allocations already warn */
> > +		warn_alloc(gfp_mask, NULL,
> >  			  "vmalloc: allocation failure: %lu bytes", real_size);
> > +	}
> >  	return NULL;
> >  }
> >    
> 
> 
> 
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