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Subject[PATCH 4/4] Hugetlb: Copy on Write support
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Hugetlb: Copy on Write support

Implement copy-on-write support for hugetlb mappings so MAP_PRIVATE can be
supported. This helps us to safely use hugetlb pages in many more
applications. The patch makes the following changes. If needed, I also have
it broken out according to the following paragraphs.

1. Add a pair of functions to set/clear write access on huge ptes. The
writable check in make_huge_pte is moved out to the caller for use by COW
later.

2. Hugetlb copy-on-write requires special case handling in the following
situations:
- copy_hugetlb_page_range() - Copied pages must be write protected so a COW
fault will be triggered (if necessary) if those pages are written to.
- find_or_alloc_huge_page() - Only MAP_SHARED pages are added to the page
cache. MAP_PRIVATE pages still need to be locked however.

3. Provide hugetlb_cow() and calls from hugetlb_fault() and hugetlb_no_page()
which handles the COW fault by making the actual copy.

4. Remove the check in hugetlbfs_file_map() so that MAP_PRIVATE mmaps will be
allowed. Make MAP_HUGETLB exempt from the depricated VM_RESERVED mapping
check.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Adam Litke <agl@us.ibm.com>
---
fs/hugetlbfs/inode.c | 3 -
include/linux/hugetlb.h | 12 +++++
mm/hugetlb.c | 115 ++++++++++++++++++++++++++++++++++++++++--------
mm/mmap.c | 4 -
4 files changed, 110 insertions(+), 24 deletions(-)
diff -upN reference/fs/hugetlbfs/inode.c current/fs/hugetlbfs/inode.c
--- reference/fs/hugetlbfs/inode.c
+++ current/fs/hugetlbfs/inode.c
@@ -100,9 +100,6 @@ static int hugetlbfs_file_mmap(struct fi
loff_t len, vma_len;
int ret;

- if ((vma->vm_flags & (VM_MAYSHARE | VM_WRITE)) == VM_WRITE)
- return -EINVAL;
-
if (vma->vm_pgoff & (HPAGE_SIZE / PAGE_SIZE - 1))
return -EINVAL;

diff -upN reference/include/linux/hugetlb.h current/include/linux/hugetlb.h
--- reference/include/linux/hugetlb.h
+++ current/include/linux/hugetlb.h
@@ -65,6 +65,18 @@ pte_t huge_ptep_get_and_clear(struct mm_
pte_t *ptep);
#endif

+#define huge_ptep_set_wrprotect(mm, addr, ptep) \
+ ptep_set_wrprotect(mm, addr, ptep)
+static inline void set_huge_ptep_writable(struct vm_area_struct *vma,
+ unsigned long address, pte_t *ptep)
+{
+ pte_t entry;
+
+ entry = pte_mkwrite(pte_mkdirty(*ptep));
+ ptep_set_access_flags(vma, address, ptep, entry, 1);
+ update_mmu_cache(vma, address, entry);
+}
+
#ifndef ARCH_HAS_HUGETLB_PREFAULT_HOOK
#define hugetlb_prefault_arch_hook(mm) do { } while (0)
#else
diff -upN reference/mm/hugetlb.c current/mm/hugetlb.c
--- reference/mm/hugetlb.c
+++ current/mm/hugetlb.c
@@ -255,11 +255,12 @@ struct vm_operations_struct hugetlb_vm_o
.nopage = hugetlb_nopage,
};

-static pte_t make_huge_pte(struct vm_area_struct *vma, struct page *page)
+static pte_t make_huge_pte(struct vm_area_struct *vma, struct page *page,
+ int writable)
{
pte_t entry;

- if (vma->vm_flags & VM_WRITE) {
+ if (writable) {
entry =
pte_mkwrite(pte_mkdirty(mk_pte(page, vma->vm_page_prot)));
} else {
@@ -277,6 +278,9 @@ int copy_hugetlb_page_range(struct mm_st
pte_t *src_pte, *dst_pte, entry;
struct page *ptepage;
unsigned long addr;
+ int cow;
+
+ cow = (vma->vm_flags & (VM_SHARED | VM_MAYWRITE)) == VM_MAYWRITE;

for (addr = vma->vm_start; addr < vma->vm_end; addr += HPAGE_SIZE) {
src_pte = huge_pte_offset(src, addr);
@@ -288,6 +292,8 @@ int copy_hugetlb_page_range(struct mm_st
spin_lock(&dst->page_table_lock);
spin_lock(&src->page_table_lock);
if (!pte_none(*src_pte)) {
+ if (cow)
+ huge_ptep_set_wrprotect(src, addr, src_pte);
entry = *src_pte;
ptepage = pte_page(entry);
get_page(ptepage);
@@ -340,7 +346,7 @@ void unmap_hugepage_range(struct vm_area
}

static struct page *find_or_alloc_huge_page(struct address_space *mapping,
- unsigned long idx)
+ unsigned long idx, int shared)
{
struct page *page;
int err;
@@ -358,26 +364,80 @@ retry:
goto out;
}

- err = add_to_page_cache(page, mapping, idx, GFP_KERNEL);
- if (err) {
- put_page(page);
- hugetlb_put_quota(mapping);
- if (err == -EEXIST)
- goto retry;
- page = NULL;
+ if (shared) {
+ err = add_to_page_cache(page, mapping, idx, GFP_KERNEL);
+ if (err) {
+ put_page(page);
+ hugetlb_put_quota(mapping);
+ if (err == -EEXIST)
+ goto retry;
+ page = NULL;
+ }
+ } else {
+ /* Caller expects a locked page */
+ lock_page(page);
}
out:
return page;
}

+static int hugetlb_cow(struct mm_struct *mm, struct vm_area_struct *vma,
+ unsigned long address, pte_t *ptep, pte_t pte)
+{
+ struct page *old_page, *new_page;
+ int i, avoidcopy;
+
+ old_page = pte_page(pte);
+
+ /* If no-one else is actually using this page, avoid the copy
+ * and just make the page writable */
+ avoidcopy = (page_count(old_page) == 1);
+ if (avoidcopy) {
+ set_huge_ptep_writable(vma, address, ptep);
+ return VM_FAULT_MINOR;
+ }
+
+ page_cache_get(old_page);
+ new_page = alloc_huge_page();
+
+ if (! new_page) {
+ page_cache_release(old_page);
+
+ /* Logically this is OOM, not a SIGBUS, but an OOM
+ * could cause the kernel to go killing other
+ * processes which won't help the hugepage situation
+ * at all (?) */
+ return VM_FAULT_SIGBUS;
+ }
+
+ spin_unlock(&mm->page_table_lock);
+ for (i = 0; i < HPAGE_SIZE/PAGE_SIZE; i++)
+ copy_user_highpage(new_page + i, old_page + i,
+ address + i*PAGE_SIZE);
+ 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);
+ page_cache_release(old_page);
+ return VM_FAULT_MINOR;
+}
+
int hugetlb_no_page(struct mm_struct *mm, struct vm_area_struct *vma,
- unsigned long address, pte_t *ptep)
+ unsigned long address, pte_t *ptep, int write_access)
{
int ret = VM_FAULT_SIGBUS;
unsigned long idx;
unsigned long size;
struct page *page;
struct address_space *mapping;
+ pte_t new_pte;

mapping = vma->vm_file->f_mapping;
idx = ((address - vma->vm_start) >> HPAGE_SHIFT)
@@ -387,10 +447,13 @@ int hugetlb_no_page(struct mm_struct *mm
* Use page lock to guard against racing truncation
* before we get page_table_lock.
*/
- page = find_or_alloc_huge_page(mapping, idx);
+ page = find_or_alloc_huge_page(mapping, idx,
+ vma->vm_flags & VM_SHARED);
if (!page)
goto out;

+ BUG_ON(!PageLocked(page));
+
spin_lock(&mm->page_table_lock);
size = i_size_read(mapping->host) >> HPAGE_SHIFT;
if (idx >= size)
@@ -401,7 +464,15 @@ int hugetlb_no_page(struct mm_struct *mm
goto backout;

add_mm_counter(mm, file_rss, HPAGE_SIZE / PAGE_SIZE);
- set_huge_pte_at(mm, address, ptep, make_huge_pte(vma, page));
+ new_pte = make_huge_pte(vma, page, ((vma->vm_flags & VM_WRITE)
+ && (vma->vm_flags & VM_SHARED)));
+ set_huge_pte_at(mm, address, ptep, new_pte);
+
+ 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);
+ }
+
spin_unlock(&mm->page_table_lock);
unlock_page(page);
out:
@@ -420,6 +491,7 @@ int hugetlb_fault(struct mm_struct *mm,
{
pte_t *ptep;
pte_t entry;
+ int ret;

ptep = huge_pte_alloc(mm, address);
if (!ptep)
@@ -427,13 +499,18 @@ int hugetlb_fault(struct mm_struct *mm,

entry = *ptep;
if (pte_none(entry))
- return hugetlb_no_page(mm, vma, address, ptep);
+ return hugetlb_no_page(mm, vma, address, ptep, write_access);

- /*
- * We could get here if another thread instantiated the pte
- * before the test above.
- */
- return VM_FAULT_MINOR;
+ 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);
+
+ return ret;
}

int follow_hugetlb_page(struct mm_struct *mm, struct vm_area_struct *vma,
diff -upN reference/mm/mmap.c current/mm/mmap.c
--- reference/mm/mmap.c
+++ current/mm/mmap.c
@@ -1077,8 +1077,8 @@ munmap_back:
error = file->f_op->mmap(file, vma);
if (error)
goto unmap_and_free_vma;
- if ((vma->vm_flags & (VM_SHARED | VM_WRITE | VM_RESERVED))
- == (VM_WRITE | VM_RESERVED)) {
+ if ((vma->vm_flags & (VM_SHARED | VM_WRITE | VM_RESERVED
+ | VM_HUGETLB)) == (VM_WRITE | VM_RESERVED)) {
printk(KERN_WARNING "program %s is using MAP_PRIVATE, "
"PROT_WRITE mmap of VM_RESERVED memory, which "
"is deprecated. Please report this to "
--
Adam Litke - (agl at us.ibm.com)
IBM Linux Technology Center

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