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Subject[PATCH] spinlock in function hugetlb_fault could be deleted
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Function hugetlb_fault needn't hold spinlock mm->page_table_lock,
because when hugetlb_fault is called:
1) mm->mmap_sem is held already;
2) hugetlb_instantiation_mutex is held by hugetlb_fault, which prevents
other threads/processes from entering this critical area. It's impossible
for other threads/processes to change the page table now.

My patch against kenel 2.6.22 deletes the spinlock statements in the
related functions.

Signed-off-by: Zhang Yanmin <yanmin.zhang@intel.com>

---
diff -Nraup linux-2.6.22/mm/hugetlb.c linux-2.6.22_hugetlb/mm/hugetlb.c
--- linux-2.6.22/mm/hugetlb.c 2007-07-09 07:32:17.000000000 +0800
+++ linux-2.6.22_hugetlb/mm/hugetlb.c 2007-07-23 15:51:49.000000000 +0800
@@ -434,12 +434,12 @@ void unmap_hugepage_range(struct vm_area
}

static int hugetlb_cow(struct mm_struct *mm, struct vm_area_struct *vma,
- unsigned long address, pte_t *ptep, pte_t pte)
+ unsigned long address, pte_t *ptep)
{
struct page *old_page, *new_page;
int avoidcopy;

- old_page = pte_page(pte);
+ old_page = pte_page(*ptep);

/* If no-one else is actually using this page, avoid the copy
* and just make the page writable */
@@ -449,28 +449,16 @@ static int hugetlb_cow(struct mm_struct
return VM_FAULT_MINOR;
}

- page_cache_get(old_page);
new_page = alloc_huge_page(vma, address);
-
if (!new_page) {
page_cache_release(old_page);
return VM_FAULT_OOM;
}

- spin_unlock(&mm->page_table_lock);
copy_huge_page(new_page, old_page, address, vma);
- spin_lock(&mm->page_table_lock);
-
- ptep = huge_pte_offset(mm, address & HPAGE_MASK);
- if (likely(pte_same(*ptep, pte))) {
- /* Break COW */
- set_huge_pte_at(mm, address, ptep,
- make_huge_pte(vma, new_page, 1));
- /* Make the old page be freed below */
- new_page = old_page;
- }
- page_cache_release(new_page);
+ set_huge_pte_at (mm, address, ptep, make_huge_pte (vma, new_page, 1));
page_cache_release(old_page);
+
return VM_FAULT_MINOR;
}

@@ -523,7 +511,6 @@ retry:
lock_page(page);
}

- spin_lock(&mm->page_table_lock);
size = i_size_read(mapping->host) >> HPAGE_SHIFT;
if (idx >= size)
goto backout;
@@ -538,16 +525,14 @@ retry:

if (write_access && !(vma->vm_flags & VM_SHARED)) {
/* Optimization, do the COW without a second fault */
- ret = hugetlb_cow(mm, vma, address, ptep, new_pte);
+ ret = hugetlb_cow(mm, vma, address, ptep);
}

- spin_unlock(&mm->page_table_lock);
unlock_page(page);
out:
return ret;

backout:
- spin_unlock(&mm->page_table_lock);
hugetlb_put_quota(mapping);
unlock_page(page);
put_page(page);
@@ -580,13 +565,8 @@ int hugetlb_fault(struct mm_struct *mm,
}

ret = VM_FAULT_MINOR;
-
- spin_lock(&mm->page_table_lock);
- /* Check for a racing update before calling hugetlb_cow */
- if (likely(pte_same(entry, *ptep)))
- if (write_access && !pte_write(entry))
- ret = hugetlb_cow(mm, vma, address, ptep, entry);
- spin_unlock(&mm->page_table_lock);
+ if (write_access && !pte_write(entry))
+ ret = hugetlb_cow(mm, vma, address, ptep);
mutex_unlock(&hugetlb_instantiation_mutex);

return ret;
-
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