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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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