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SubjectRe: [PATCH 2/3] thp: move release mmap_sem lock out of khugepaged_alloc_page
On Wed, 12 Sep 2012 20:56:41 +0800
Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com> wrote:

> To make the code more clear, move release the lock out of khugepaged_alloc_page
>
> ...
>
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -1854,11 +1854,6 @@ static struct page
> *hpage = alloc_hugepage_vma(khugepaged_defrag(), vma, address,
> node, __GFP_OTHER_NODE);
>
> - /*
> - * After allocating the hugepage, release the mmap_sem read lock in
> - * preparation for taking it in write mode.
> - */
> - up_read(&mm->mmap_sem);
> if (unlikely(!*hpage)) {
> count_vm_event(THP_COLLAPSE_ALLOC_FAILED);
> *hpage = ERR_PTR(-ENOMEM);
> @@ -1905,7 +1900,6 @@ static struct page
> struct vm_area_struct *vma, unsigned long address,
> int node)
> {
> - up_read(&mm->mmap_sem);
> VM_BUG_ON(!*hpage);
> return *hpage;
> }
> @@ -1931,8 +1925,14 @@ static void collapse_huge_page(struct mm_struct *mm,
>
> VM_BUG_ON(address & ~HPAGE_PMD_MASK);
>
> - /* release the mmap_sem read lock. */
> new_page = khugepaged_alloc_page(hpage, mm, vma, address, node);
> +
> + /*
> + * After allocating the hugepage, release the mmap_sem read lock in
> + * preparation for taking it in write mode.
> + */
> + up_read(&mm->mmap_sem);
> +
> if (!new_page)
> return;

Well that's a pretty minor improvement: one still has to go off on a
big hunt to locate the matching down_read().

And the patch will increase mmap_sem hold times by a teeny amount. Do
we really want to do this?



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