[PATCH v10 04/21] Allow page fault handlers to perform the COW

Matthew Wilcox
2014-08-27T04:36:08Z

Currently COW of an XIP file is done by first bringing in a read-only
mapping, then retrying the fault and copying the page. It is much more
efficient to tell the fault handler that a COW is being attempted (by
passing in the pre-allocated page in the vm_fault structure), and allow
the handler to perform the COW operation itself.

The handler cannot insert the page itself if there is already a read-only
mapping at that address, so allow the handler to return VM_FAULT_LOCKED
and set the fault_page to be NULL. This indicates to the MM code that
the i_mmap_mutex is held instead of the page lock.

Signed-off-by: Matthew Wilcox <matthew.r.wilcox@intel.com>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
---
include/linux/mm.h | 1 +
mm/memory.c | 33 ++++++++++++++++++++++++---------
2 files changed, 25 insertions(+), 9 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 8981cc8..0a47817 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -208,6 +208,7 @@ struct vm_fault {
pgoff_t pgoff; /* Logical page offset based on vma */
void __user *virtual_address; /* Faulting virtual address */

+ struct page *cow_page; /* Handler may choose to COW */
struct page *page; /* ->fault handlers should return a
* page here, unless VM_FAULT_NOPAGE
* is set (which is also implied by
diff --git a/mm/memory.c b/mm/memory.c
index adeac30..3368785 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -2000,6 +2000,7 @@ static int do_page_mkwrite(struct vm_area_struct *vma, struct page *page,
vmf.pgoff = page->index;
vmf.flags = FAULT_FLAG_WRITE|FAULT_FLAG_MKWRITE;
vmf.page = page;
+ vmf.cow_page = NULL;

ret = vma->vm_ops->page_mkwrite(vma, &vmf);
if (unlikely(ret & (VM_FAULT_ERROR | VM_FAULT_NOPAGE)))
@@ -2698,7 +2699,8 @@ oom:
* See filemap_fault() and __lock_page_retry().
*/
static int __do_fault(struct vm_area_struct *vma, unsigned long address,
- pgoff_t pgoff, unsigned int flags, struct page **page)
+ pgoff_t pgoff, unsigned int flags,
+ struct page *cow_page, struct page **page)
{
struct vm_fault vmf;
int ret;
@@ -2707,10 +2709,13 @@ static int __do_fault(struct vm_area_struct *vma, unsigned long address,
vmf.pgoff = pgoff;
vmf.flags = flags;
vmf.page = NULL;
+ vmf.cow_page = cow_page;

ret = vma->vm_ops->fault(vma, &vmf);
if (unlikely(ret & (VM_FAULT_ERROR | VM_FAULT_NOPAGE | VM_FAULT_RETRY)))
return ret;
+ if (!vmf.page)
+ goto out;

if (unlikely(PageHWPoison(vmf.page))) {
if (ret & VM_FAULT_LOCKED)
@@ -2724,6 +2729,7 @@ static int __do_fault(struct vm_area_struct *vma, unsigned long address,
else
VM_BUG_ON_PAGE(!PageLocked(vmf.page), vmf.page);

+ out:
*page = vmf.page;
return ret;
}
@@ -2897,7 +2903,7 @@ static int do_read_fault(struct mm_struct *mm, struct vm_area_struct *vma,
pte_unmap_unlock(pte, ptl);
}

- ret = __do_fault(vma, address, pgoff, flags, &fault_page);
+ ret = __do_fault(vma, address, pgoff, flags, NULL, &fault_page);
if (unlikely(ret & (VM_FAULT_ERROR | VM_FAULT_NOPAGE | VM_FAULT_RETRY)))
return ret;

@@ -2937,26 +2943,35 @@ static int do_cow_fault(struct mm_struct *mm, struct vm_area_struct *vma,
return VM_FAULT_OOM;
}

- ret = __do_fault(vma, address, pgoff, flags, &fault_page);
+ ret = __do_fault(vma, address, pgoff, flags, new_page, &fault_page);
if (unlikely(ret & (VM_FAULT_ERROR | VM_FAULT_NOPAGE | VM_FAULT_RETRY)))
goto uncharge_out;

- copy_user_highpage(new_page, fault_page, address, vma);
+ if (fault_page)
+ copy_user_highpage(new_page, fault_page, address, vma);
__SetPageUptodate(new_page);

pte = pte_offset_map_lock(mm, pmd, address, &ptl);
if (unlikely(!pte_same(*pte, orig_pte))) {
pte_unmap_unlock(pte, ptl);
- unlock_page(fault_page);
- page_cache_release(fault_page);
+ if (fault_page) {
+ unlock_page(fault_page);
+ page_cache_release(fault_page);
+ } else {
+ mutex_unlock(&vma->vm_file->f_mapping->i_mmap_mutex);
+ }
goto uncharge_out;
}
do_set_pte(vma, address, new_page, pte, true, true);
mem_cgroup_commit_charge(new_page, memcg, false);
lru_cache_add_active_or_unevictable(new_page, vma);
pte_unmap_unlock(pte, ptl);
- unlock_page(fault_page);
- page_cache_release(fault_page);
+ if (fault_page) {
+ unlock_page(fault_page);
+ page_cache_release(fault_page);
+ } else {
+ mutex_unlock(&vma->vm_file->f_mapping->i_mmap_mutex);
+ }
return ret;
uncharge_out:
mem_cgroup_cancel_charge(new_page, memcg);
@@ -2975,7 +2990,7 @@ static int do_shared_fault(struct mm_struct *mm, struct vm_area_struct *vma,
int dirtied = 0;
int ret, tmp;

- ret = __do_fault(vma, address, pgoff, flags, &fault_page);
+ ret = __do_fault(vma, address, pgoff, flags, NULL, &fault_page);
if (unlikely(ret & (VM_FAULT_ERROR | VM_FAULT_NOPAGE | VM_FAULT_RETRY)))
return ret;

--
2.0.0