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SubjectRe: [RFC PATCH 3/3] mm/map_contig: Add mmap(MAP_CONTIG) support
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On 10/12/2017 07:16 AM, 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.
>
> Signed-off-by: Mike Kravetz <mike.kravetz@oracle.com>
> ---
> 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;

Would it be GFP_HIGHUSER_MOVABLE instead ? Why __GFP_ZERO ? If its
coming from Buddy, every thing should have already been zeroed out
in there. Am I missing something ?

> + 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);

This is where I was experimenting for requests beyond MAX_ORDER
with alloc_contig_range().

> + 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);

Interesting and this should be kept there a little longer if we would like
to support expansion of this VMA to the next power of 2 which we originally
allocated for any way.

> + }
> +
> + 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;
> + }
> +

You wanted to have this outside of mmap_sem lock right ?

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