Re: [RFC PATCH 3/3] mm/map_contig: Add mmap(MAP_CONTIG) support

[Michal Hocko]

On Wed 11-10-17 18:46:11, Mike Kravetz wrote:
> Add new MAP_CONTIG flag to mmap system call.  Check for flag in normal
> mmap flag processing.  If present, pre-allocate a contiguous set of
> pages to back the mapping.  These pages will be used a fault time, and
> the MAP_CONTIG flag implies populating the mapping at the mmap time.

I have only briefly read through the previous discussion and it is still
not clear to me _why_ we want such a interface. I didn't give it much
time yet but I do not think this is a good idea at all. Why? Do we want
any user to simply consume larger order memory blocks? What would
prevent from that? Also why should even userspace care about larger
memory blocks? We have huge pages (be it preallocated or transparent)
for that purpose already. Why should we add yet another another type
of physically contiguous memory. What is the guaratee of such a mapping.
Does the memory always stays contiguous? How much contiguous it will be?
Who is going to use such an interface? And probably many other
questions...

> Signed-off-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
> ---
>  include/uapi/asm-generic/mman.h |  1 +
>  mm/mmap.c                       | 94 +++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 95 insertions(+)
> 
> diff --git a/include/uapi/asm-generic/mman.h b/include/uapi/asm-generic/mman.h
> index 7162cd4cca73..e8046b4c4ac4 100644
> --- a/include/uapi/asm-generic/mman.h
> +++ b/include/uapi/asm-generic/mman.h
> @@ -12,6 +12,7 @@
>  #define MAP_NONBLOCK	0x10000		/* do not block on IO */
>  #define MAP_STACK	0x20000		/* give out an address that is best suited for process/thread stacks */
>  #define MAP_HUGETLB	0x40000		/* create a huge page mapping */
> +#define MAP_CONTIG	0x80000		/* back with contiguous pages */
>  
>  /* Bits [26:31] are reserved, see mman-common.h for MAP_HUGETLB usage */
>  
> diff --git a/mm/mmap.c b/mm/mmap.c
> index 680506faceae..aee7917ee073 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -167,6 +167,16 @@ static struct vm_area_struct *remove_vma(struct vm_area_struct *vma)
>  {
>  	struct vm_area_struct *next = vma->vm_next;
>  
> +	if (vma->vm_flags & VM_CONTIG) {
> +		/*
> +		 * Do any necessary clean up when freeing a vma backed
> +		 * by a contiguous allocation.
> +		 *
> +		 * Not very useful in it's present form.
> +		 */
> +		VM_BUG_ON(!vma->vm_private_data);
> +		vma->vm_private_data = NULL;
> +	}
>  	might_sleep();
>  	if (vma->vm_ops && vma->vm_ops->close)
>  		vma->vm_ops->close(vma);
> @@ -1378,6 +1388,18 @@ unsigned long do_mmap(struct file *file, unsigned long addr,
>  	vm_flags |= calc_vm_prot_bits(prot, pkey) | calc_vm_flag_bits(flags) |
>  			mm->def_flags | VM_MAYREAD | VM_MAYWRITE | VM_MAYEXEC;
>  
> +	/*
> +	 * MAP_CONTIG has some restrictions,
> +	 * and also implies additional mmap and vma flags.
> +	 */
> +	if (flags & MAP_CONTIG) {
> +		if (!(flags & MAP_ANONYMOUS))
> +			return -EINVAL;
> +
> +		flags |= MAP_POPULATE | MAP_LOCKED;
> +		vm_flags |= (VM_CONTIG | VM_LOCKED | VM_DONTEXPAND);
> +	}
> +
>  	if (flags & MAP_LOCKED)
>  		if (!can_do_mlock())
>  			return -EPERM;
> @@ -1547,6 +1569,71 @@ SYSCALL_DEFINE1(old_mmap, struct mmap_arg_struct __user *, arg)
>  #endif /* __ARCH_WANT_SYS_OLD_MMAP */
>  
>  /*
> + * Attempt to allocate a contiguous range of pages to back the
> + * specified vma.  vm_private_data is used as a 'pointer' to the
> + * allocated pages.  Larger requests and more fragmented memory
> + * make the allocation more likely to fail.  So, caller must deal
> + * with this situation.
> + */
> +static long __alloc_vma_contig_range(struct vm_area_struct *vma)
> +{
> +	gfp_t gfp = GFP_HIGHUSER | __GFP_ZERO;
> +	unsigned long order;
> +
> +	VM_BUG_ON_VMA(vma->vm_private_data != NULL, vma);
> +	order = get_order(vma->vm_end - vma->vm_start);
> +
> +	/*
> +	 * FIXME - Incomplete implementation.  For now, just handle
> +	 * allocations < MAX_ORDER in size.  However, this should really
> +	 * handle arbitrary size allocations.
> +	 */
> +	if (order >= MAX_ORDER)
> +		return -ENOMEM;
> +
> +	vma->vm_private_data = alloc_pages_vma(gfp, order, vma, vma->vm_start,
> +						numa_node_id(), false);
> +	if (!vma->vm_private_data)
> +		return -ENOMEM;
> +
> +	/*
> +	 * split large allocation so it can be treated as individual
> +	 * pages when populating the mapping and at unmap time.
> +	 */
> +	if (order) {
> +		unsigned long vma_pages = (vma->vm_end - vma->vm_start) /
> +								PAGE_SIZE;
> +		unsigned long order_pages = 1 << order;
> +		unsigned long i;
> +		struct page *page = vma->vm_private_data;
> +
> +		split_page((struct page *)vma->vm_private_data, order);
> +
> +		/*
> +		 * 'order' rounds up size of vma to next power of 2.  We
> +		 * will not need/use the extra pages so free them now.
> +		 */
> +		for (i = vma_pages; i < order_pages; i++)
> +			put_page(page + i);
> +	}
> +
> +	return 0;
> +}
> +
> +static void __free_vma_contig_range(struct vm_area_struct *vma)
> +{
> +	struct page *page = vma->vm_private_data;
> +	unsigned long n_pages = (vma->vm_end - vma->vm_start) / PAGE_SIZE;
> +	unsigned long i;
> +
> +	if (!page)
> +		return;
> +
> +	for (i = 0; i < n_pages; i++)
> +		put_page(page + i);
> +}
> +
> +/*
>   * Some shared mappigns will want the pages marked read-only
>   * to track write events. If so, we'll downgrade vm_page_prot
>   * to the private version (using protection_map[] without the
> @@ -1669,6 +1756,12 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
>  	vma->vm_pgoff = pgoff;
>  	INIT_LIST_HEAD(&vma->anon_vma_chain);
>  
> +	if (vm_flags & VM_CONTIG) {
> +		error = __alloc_vma_contig_range(vma);
> +		if (error)
> +			goto free_vma;
> +	}
> +
>  	if (file) {
>  		if (vm_flags & VM_DENYWRITE) {
>  			error = deny_write_access(file);
> @@ -1758,6 +1851,7 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
>  	if (vm_flags & VM_DENYWRITE)
>  		allow_write_access(file);
>  free_vma:
> +	__free_vma_contig_range(vma);
>  	kmem_cache_free(vm_area_cachep, vma);
>  unacct_error:
>  	if (charged)
> -- 
> 2.13.6
> 
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-- 
Michal Hocko
SUSE Labs