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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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