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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: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Adam Litke <agl@us.ibm.com>
Cc: William Lee Irwin III <wli@holomorphy.com>
Cc: "Seth, Rohit" <rohit.seth@intel.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>
This commit is contained in:
David Gibson 2006-01-06 00:10:44 -08:00 committed by Linus Torvalds
parent 86e5216f8d
commit 1e8f889b10
2 changed files with 108 additions and 22 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

3
fs/hugetlbfs/inode.c
View File

@ -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;

127
mm/hugetlb.c
View File

@ -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;
@ -364,26 +382,80 @@ 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)
@ -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,