On Mon, Nov 07, 2005 at 03:38:53PM -0600, Adam Litke wrote:
> [RFC] COW for hugepages
> (Patch originally from David Gibson <[email protected]>)
Just realised there's at least one seriouss bug in the patch -
find_or_alloc_huge_page() can return a non-locked page when demand
faulting a PRIVATE mapping. That will cause the unlock_page() further
down to BUG(). Here's a corrected version.
This patch implements copy-on-write for hugepages, hence allowing
MAP_SHARED mappings of hugetlbfs.
This is chiefly useful for cases where we want to use hugepages
"automatically" - that is to map hugepages without the knowledge of
the code in the final application (either via kernel hooks, or with
LD_PRELOAD). We can use various heuristics to determine when
hugepages might be a good idea, but changing the semantics of
anonymous memory from MAP_PRIVATE to MAP_SHARED without the app's
knowledge is clearly wrong.
Index: working-2.6/fs/hugetlbfs/inode.c
===================================================================
--- working-2.6.orig/fs/hugetlbfs/inode.c 2005-11-08 14:04:59.000000000 +1100
+++ working-2.6/fs/hugetlbfs/inode.c 2005-11-08 14:05:04.000000000 +1100
@@ -100,9 +100,6 @@
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;
Index: working-2.6/include/linux/hugetlb.h
===================================================================
--- working-2.6.orig/include/linux/hugetlb.h 2005-11-08 14:04:59.000000000 +1100
+++ working-2.6/include/linux/hugetlb.h 2005-11-08 14:05:04.000000000 +1100
@@ -65,6 +65,17 @@
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
Index: working-2.6/mm/hugetlb.c
===================================================================
--- working-2.6.orig/mm/hugetlb.c 2005-11-08 14:05:04.000000000 +1100
+++ working-2.6/mm/hugetlb.c 2005-11-08 14:21:13.000000000 +1100
@@ -255,11 +255,12 @@
.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 @@
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 @@
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 @@
}
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;
@@ -359,26 +365,81 @@
return NULL;
}
- 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);
}
return page;
}
-int hugetlb_no_page(struct mm_struct *mm, struct vm_area_struct *vma,
- unsigned long address, pte_t *ptep)
+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;
+}
+
+static int hugetlb_no_page(struct mm_struct *mm, struct vm_area_struct *vma,
+ unsigned long address, pte_t *ptep,
+ int write_access)
{
int ret;
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)
@@ -386,10 +447,13 @@
/* This returns a locked page, which keeps us safe in the
* event of a race with truncate() */
- page = find_or_alloc_huge_page(mapping, idx);
+ page = find_or_alloc_huge_page(mapping, idx,
+ vma->vm_flags & VM_SHARED);
if (!page)
return VM_FAULT_SIGBUS;
+ BUG_ON(!PageLocked(page));
+
spin_lock(&mm->page_table_lock);
ret = VM_FAULT_SIGBUS;
@@ -405,7 +469,16 @@
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);
@@ -426,6 +499,7 @@
{
pte_t *ptep;
pte_t entry;
+ int ret;
ptep = huge_pte_alloc(mm, address);
if (! ptep)
@@ -434,12 +508,20 @@
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 */
+ ret = VM_FAULT_MINOR;
- return VM_FAULT_MINOR;
+ spin_lock(&mm->page_table_lock);
+
+ if (likely(pte_same(entry, *ptep)))
+ /* pte could have changed before we grabbed the lock */
+ 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,
Index: working-2.6/mm/mmap.c
===================================================================
--- working-2.6.orig/mm/mmap.c 2005-11-08 14:04:59.000000000 +1100
+++ working-2.6/mm/mmap.c 2005-11-08 14:05:04.000000000 +1100
@@ -1076,7 +1076,7 @@
error = file->f_op->mmap(file, vma);
if (error)
goto unmap_and_free_vma;
- if ((vma->vm_flags & (VM_SHARED | 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 "
--
David Gibson | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au | minimalist, thank you. NOT _the_ _other_
| _way_ _around_!
http://www.ozlabs.org/~dgibson
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