Re: [PATCH V4] mm/thp: Allocate transparent hugepages on local node

[Andrew Morton <akpm@linux-foundation.org>]

On Tue, 20 Jan 2015 17:04:31 +0530 "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com> wrote:

> This make sure that we try to allocate hugepages from local node if
> allowed by mempolicy. If we can't, we fallback to small page allocation
> based on mempolicy. This is based on the observation that allocating pages
> on local node is more beneficial than allocating hugepages on remote
> node.
> 
> With this patch applied we may find transparent huge page allocation
> failures if the current node doesn't have enough freee hugepages.
> Before this patch such failures result in us retrying the allocation on
> other nodes in the numa node mask.
> 
>  
>  /**
> + * alloc_hugepage_vma: Allocate a hugepage for a VMA
> + * @gfp:
> + *   %GFP_USER	  user allocation.
> + *   %GFP_KERNEL  kernel allocations,
> + *   %GFP_HIGHMEM highmem/user allocations,
> + *   %GFP_FS	  allocation should not call back into a file system.
> + *   %GFP_ATOMIC  don't sleep.
> + *
> + * @vma:   Pointer to VMA or NULL if not available.
> + * @addr:  Virtual Address of the allocation. Must be inside the VMA.
> + * @order: Order of the hugepage for gfp allocation.
> + *
> + * This functions allocate a huge page from the kernel page pool and applies
> + * a NUMA policy associated with the VMA or the current process.
> + * For policy other than %MPOL_INTERLEAVE, we make sure we allocate hugepage
> + * only from the current node if the current node is part of the node mask.
> + * If we can't allocate a hugepage we fail the allocation and don' try to fallback
> + * to other nodes in the node mask. If the current node is not part of node mask
> + * or if the NUMA policy is MPOL_INTERLEAVE we use the allocator that can
> + * fallback to nodes in the policy node mask.
> + *
> + * When VMA is not NULL caller must hold down_read on the mmap_sem of the
> + * mm_struct of the VMA to prevent it from going away. Should be used for
> + * all allocations for pages that will be mapped into
> + * user space. Returns NULL when no page can be allocated.
> + *
> + * Should be called with the mm_sem of the vma hold.

That's a pretty cruddy sentence, isn't it?  Copied from
alloc_pages_vma().  "vma->vm_mm->mmap_sem" would be better.

And it should tell us whether mmap_sem required a down_read or a
down_write.  What purpose is it serving?

> + *
> + */
> +struct page *alloc_hugepage_vma(gfp_t gfp, struct vm_area_struct *vma,
> +				unsigned long addr, int order)

This pointlessly bloats the kernel if CONFIG_TRANSPARENT_HUGEPAGE=n?



--- a/mm/mempolicy.c~mm-thp-allocate-transparent-hugepages-on-local-node-fix
+++ a/mm/mempolicy.c
@@ -2030,6 +2030,7 @@ retry_cpuset:
 	return page;
 }
 
+#ifdef CONFIG_TRANSPARENT_HUGEPAGE
 /**
  * alloc_hugepage_vma: Allocate a hugepage for a VMA
  * @gfp:
@@ -2057,7 +2058,7 @@ retry_cpuset:
  * all allocations for pages that will be mapped into
  * user space. Returns NULL when no page can be allocated.
  *
- * Should be called with the mm_sem of the vma hold.
+ * Should be called with vma->vm_mm->mmap_sem held.
  *
  */
 struct page *alloc_hugepage_vma(gfp_t gfp, struct vm_area_struct *vma,
@@ -2099,6 +2100,7 @@ alloc_with_fallback:
 	 */
 	return alloc_pages_vma(gfp, order, vma, addr, node);
 }
+#endif
 
 /**
  * 	alloc_pages_current - Allocate pages.
_

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