[PATCH] mm: tracking shared dirty pages
Tracking of dirty pages in shared writeable mmap()s.
The idea is simple: write protect clean shared writeable pages, catch the
write-fault, make writeable and set dirty. On page write-back clean all the
PTE dirty bits and write protect them once again.
The implementation is a tad harder, mainly because the default
backing_dev_info capabilities were too loosely maintained. Hence it is not
enough to test the backing_dev_info for cap_account_dirty.
The current heuristic is as follows, a VMA is eligible when:
- its shared writeable
(vm_flags & (VM_WRITE|VM_SHARED)) == (VM_WRITE|VM_SHARED)
- it is not a 'special' mapping
(vm_flags & (VM_PFNMAP|VM_INSERTPAGE)) == 0
- the backing_dev_info is cap_account_dirty
mapping_cap_account_dirty(vma->vm_file->f_mapping)
- f_op->mmap() didn't change the default page protection
Page from remap_pfn_range() are explicitly excluded because their COW
semantics are already horrid enough (see vm_normal_page() in do_wp_page()) and
because they don't have a backing store anyway.
mprotect() is taught about the new behaviour as well. However it overrides
the last condition.
Cleaning the pages on write-back is done with page_mkclean() a new rmap call.
It can be called on any page, but is currently only implemented for mapped
pages, if the page is found the be of a VMA that accounts dirty pages it will
also wrprotect the PTE.
Finally, in fs/buffers.c:try_to_free_buffers(); remove clear_page_dirty() from
under ->private_lock. This seems to be safe, since ->private_lock is used to
serialize access to the buffers, not the page itself. This is needed because
clear_page_dirty() will call into page_mkclean() and would thereby violate
locking order.
[dhowells@redhat.com: Provide a page_mkclean() implementation for NOMMU]
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Hugh Dickins <hugh@veritas.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>
diff --git a/mm/memory.c b/mm/memory.c
index 109e986..fa941b1 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -1458,14 +1458,19 @@
{
struct page *old_page, *new_page;
pte_t entry;
- int reuse, ret = VM_FAULT_MINOR;
+ int reuse = 0, ret = VM_FAULT_MINOR;
+ struct page *dirty_page = NULL;
old_page = vm_normal_page(vma, address, orig_pte);
if (!old_page)
goto gotten;
- if (unlikely((vma->vm_flags & (VM_SHARED|VM_WRITE)) ==
- (VM_SHARED|VM_WRITE))) {
+ /*
+ * Only catch write-faults on shared writable pages, read-only
+ * shared pages can get COWed by get_user_pages(.write=1, .force=1).
+ */
+ if (unlikely((vma->vm_flags & (VM_WRITE|VM_SHARED)) ==
+ (VM_WRITE|VM_SHARED))) {
if (vma->vm_ops && vma->vm_ops->page_mkwrite) {
/*
* Notify the address space that the page is about to
@@ -1494,13 +1499,12 @@
if (!pte_same(*page_table, orig_pte))
goto unlock;
}
-
+ dirty_page = old_page;
+ get_page(dirty_page);
reuse = 1;
} else if (PageAnon(old_page) && !TestSetPageLocked(old_page)) {
reuse = can_share_swap_page(old_page);
unlock_page(old_page);
- } else {
- reuse = 0;
}
if (reuse) {
@@ -1566,6 +1570,10 @@
page_cache_release(old_page);
unlock:
pte_unmap_unlock(page_table, ptl);
+ if (dirty_page) {
+ set_page_dirty(dirty_page);
+ put_page(dirty_page);
+ }
return ret;
oom:
if (old_page)
@@ -2098,6 +2106,7 @@
unsigned int sequence = 0;
int ret = VM_FAULT_MINOR;
int anon = 0;
+ struct page *dirty_page = NULL;
pte_unmap(page_table);
BUG_ON(vma->vm_flags & VM_PFNMAP);
@@ -2192,6 +2201,10 @@
} else {
inc_mm_counter(mm, file_rss);
page_add_file_rmap(new_page);
+ if (write_access) {
+ dirty_page = new_page;
+ get_page(dirty_page);
+ }
}
} else {
/* One of our sibling threads was faster, back out. */
@@ -2204,6 +2217,10 @@
lazy_mmu_prot_update(entry);
unlock:
pte_unmap_unlock(page_table, ptl);
+ if (dirty_page) {
+ set_page_dirty(dirty_page);
+ put_page(dirty_page);
+ }
return ret;
oom:
page_cache_release(new_page);