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Commit 1e8f889b authored by David Gibson's avatar David Gibson Committed by Linus Torvalds
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[PATCH] 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: default avatarDavid Gibson <david@gibson.dropbear.id.au>
Signed-off-by: default avatarAdam Litke <agl@us.ibm.com>
Cc: William Lee Irwin III <wli@holomorphy.com>
Cc: "Seth, Rohit" <rohit.seth@intel.com>
Signed-off-by: default avatarAndrew Morton <akpm@osdl.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@osdl.org>
parent 86e5216f

fs/hugetlbfs/inode.c

+0 −3
Original line number Diff line number Diff line
@@ -100,9 +100,6 @@ static int hugetlbfs_file_mmap(struct file *file, struct vm_area_struct *vma) 
	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;

mm/hugetlb.c

+108 −19
Original line number Diff line number Diff line
@@ -261,11 +261,12 @@ struct vm_operations_struct hugetlb_vm_ops = { 
	.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 {
@@ -277,12 +278,27 @@ static pte_t make_huge_pte(struct vm_area_struct *vma, struct page *page) 
	return entry;

}



static 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);

	lazy_mmu_prot_update(entry);

}





int copy_hugetlb_page_range(struct mm_struct *dst, struct mm_struct *src,

			    struct vm_area_struct *vma)

{

	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);
@@ -294,6 +310,8 @@ int copy_hugetlb_page_range(struct mm_struct *dst, struct mm_struct *src, 
		spin_lock(&dst->page_table_lock);

		spin_lock(&src->page_table_lock);

		if (!pte_none(*src_pte)) {

			if (cow)

				ptep_set_wrprotect(src, addr, src_pte);

			entry = *src_pte;

			ptepage = pte_page(entry);

			get_page(ptepage);
@@ -346,7 +364,7 @@ void unmap_hugepage_range(struct vm_area_struct *vma, unsigned long start, 
}



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;
@@ -364,6 +382,7 @@ static struct page *find_or_alloc_huge_page(struct address_space *mapping, 
		goto out;

	}



	if (shared) {

		err = add_to_page_cache(page, mapping, idx, GFP_KERNEL);

		if (err) {

			put_page(page);
@@ -372,18 +391,71 @@ static struct page *find_or_alloc_huge_page(struct address_space *mapping, 
				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)
@@ -393,10 +465,13 @@ int hugetlb_no_page(struct mm_struct *mm, struct vm_area_struct *vma, 
	 * 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)
@@ -407,7 +482,15 @@ int hugetlb_no_page(struct mm_struct *mm, struct vm_area_struct *vma, 
		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:
@@ -426,6 +509,7 @@ int hugetlb_fault(struct mm_struct *mm, struct vm_area_struct *vma, 
{

	pte_t *ptep;

	pte_t entry;

	int ret;



	ptep = huge_pte_alloc(mm, address);

	if (!ptep)
@@ -433,13 +517,18 @@ int hugetlb_fault(struct mm_struct *mm, struct vm_area_struct *vma, 


	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,