Re: [PATCH 1/5] mm: tracking shared dirty pages

Peter Zijlstra
2006-06-23T23:54:35Z

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 fudges
the last.

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
trackable VMA it will also wrprotect the PTE.

Bah Bah Bah, why didn't the page_mkwrite() patch re-protect clean pages?
And is it a Bad-Thing (tm) that that can happen now?

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.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
---
Changes in -v12

- Linus suggested a nice replacement for the beast condition

Changes in -v11

- small cleanups
- pulled page_mkclean back under mapping_cap_account_dirty()

Changes in -v10

- 2.6.17-mm1
- Drop the ugly duckling pgprotting, Hugh suggested resetting the
vma->vm_page_prot when f_op->mmap() didn't modify it. If it were
modified we're not interested anyway.
- abandon is_shared_writable() because its actually spelled writeable
and it didn't actually mean that any more.
- Comments all round.

Changes in -v9

- respin against latest -mm.

Changes in -v8

- access_process_vm() and other force users of get_user_pages() can
induce COW of read-only shared mappings.

Changes in -v7

- changed is_shared_writable() to exclude VM_PFNMAP'ed regions.
- Hugh's tiresome problem wasn't fully solved, now using the ugly duckling
method.

Changes in -v6

- make page_mkclean_one() modify the pte more like change_pte_range()
(suggested by Christoph Lameter)
- made is_shared_writable() take vm_flags, it now resembles is_cow_mapping().
- fixed the mprotect() bug (spotted by Hugh Dickins)
- hopefully fixed the tiresome issue of do_mmap_pgoff() trampling on
driver specific vm_page_prot settings (spotted by Hugh Dickins)
- made a new version of the page_mkwrite() patch to go on top of all this.
This so that Linus could merge this very early on in 2.6.18.

Changes in -v5

- rename page_wrprotect() to page_mkclean() (suggested by Nick Piggin)
- added comment to test_clear_page_dirty() (Andrew Morton)
- cleanup page_wrprotect() (Andrew Morton)
- renamed VM_SharedWritable() to is_shared_writable()
- fs/buffers.c try_to_free_buffers(): remove clear_page_dirty() from under
->private_lock. This seems to be save, since ->private_lock is used to
serialize access to the buffers, not the page itself.
- rebased on top of David Howells' page_mkwrite() patch.

Changes in -v4:

- small cleanup as suggested by Christoph Lameter.

Changes in -v3:

- move set_page_dirty() outside pte lock (suggested by Christoph Lameter)

Changes in -v2:

- only wrprotect pages from dirty capable mappings. (Nick Piggin)
- move the writefault handling from do_wp_page() into handle_pte_fault().
(Nick Piggin)
- revert to the old install_page interface. (Nick Piggin)
- also clear the pte dirty bit when we make pages read-only again.
(spotted by Rik van Riel)
- make page_wrprotect() return the number of reprotected ptes.

fs/buffer.c | 2 -
include/linux/mm.h | 34 ++++++++++++++++++++++++
include/linux/rmap.h | 8 +++++
mm/memory.c | 29 ++++++++++++++++-----
mm/mmap.c | 10 ++-----
mm/mprotect.c | 22 ++++++----------
mm/page-writeback.c | 15 ++++++++--
mm/rmap.c | 70 +++++++++++++++++++++++++++++++++++++++++++++++++++
8 files changed, 161 insertions(+), 29 deletions(-)

Index: 2.6-mm/mm/memory.c
===================================================================
--- 2.6-mm.orig/mm/memory.c 2006-06-24 01:39:29.000000000 +0200
+++ 2.6-mm/mm/memory.c 2006-06-24 01:40:20.000000000 +0200
@@ -1458,14 +1458,19 @@ static int do_wp_page(struct mm_struct *
{
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 @@ static int do_wp_page(struct mm_struct *
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 @@ gotten:
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 @@ static int do_no_page(struct mm_struct *
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 @@ retry:
} 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 @@ retry:
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);
Index: 2.6-mm/mm/mmap.c
===================================================================
--- 2.6-mm.orig/mm/mmap.c 2006-06-24 01:39:29.000000000 +0200
+++ 2.6-mm/mm/mmap.c 2006-06-24 01:40:20.000000000 +0200
@@ -1090,12 +1090,6 @@ munmap_back:
goto free_vma;
}

- /* Don't make the VMA automatically writable if it's shared, but the
- * backer wishes to know when pages are first written to */
- if (vma->vm_ops && vma->vm_ops->page_mkwrite)
- vma->vm_page_prot =
- protection_map[vm_flags & (VM_READ|VM_WRITE|VM_EXEC)];
-
/* We set VM_ACCOUNT in a shared mapping's vm_flags, to inform
* shmem_zero_setup (perhaps called through /dev/zero's ->mmap)
* that memory reservation must be checked; but that reservation
@@ -1113,6 +1107,10 @@ munmap_back:
pgoff = vma->vm_pgoff;
vm_flags = vma->vm_flags;

+ if (vma_wants_writenotify(vma))
+ vma->vm_page_prot =
+ protection_map[vm_flags & (VM_READ|VM_WRITE|VM_EXEC)];
+
if (!file || !vma_merge(mm, prev, addr, vma->vm_end,
vma->vm_flags, NULL, file, pgoff, vma_policy(vma))) {
file = vma->vm_file;
Index: 2.6-mm/mm/mprotect.c
===================================================================
--- 2.6-mm.orig/mm/mprotect.c 2006-06-24 01:39:29.000000000 +0200
+++ 2.6-mm/mm/mprotect.c 2006-06-24 01:40:20.000000000 +0200
@@ -123,8 +123,7 @@ mprotect_fixup(struct vm_area_struct *vm
unsigned long oldflags = vma->vm_flags;
long nrpages = (end - start) >> PAGE_SHIFT;
unsigned long charged = 0;
- unsigned int mask;
- pgprot_t newprot;
+ unsigned int mask = VM_READ|VM_WRITE|VM_EXEC|VM_SHARED;
pgoff_t pgoff;
int error;

@@ -176,24 +175,21 @@ mprotect_fixup(struct vm_area_struct *vm
}

success:
- /* Don't make the VMA automatically writable if it's shared, but the
- * backer wishes to know when pages are first written to */
- mask = VM_READ|VM_WRITE|VM_EXEC|VM_SHARED;
- if (vma->vm_ops && vma->vm_ops->page_mkwrite)
- mask &= ~VM_SHARED;
-
- newprot = protection_map[newflags & mask];
-
/*
* vm_flags and vm_page_prot are protected by the mmap_sem
* held in write mode.
*/
vma->vm_flags = newflags;
- vma->vm_page_prot = newprot;
+ vma->vm_page_prot = protection_map[newflags & mask];
+ if (vma_wants_writenotify(vma)) {
+ mask &= ~VM_SHARED;
+ vma->vm_page_prot = protection_map[newflags & mask];
+ }
+
if (is_vm_hugetlb_page(vma))
- hugetlb_change_protection(vma, start, end, newprot);
+ hugetlb_change_protection(vma, start, end, vma->vm_page_prot);
else
- change_protection(vma, start, end, newprot);
+ change_protection(vma, start, end, vma->vm_page_prot);
vm_stat_account(mm, oldflags, vma->vm_file, -nrpages);
vm_stat_account(mm, newflags, vma->vm_file, nrpages);
return 0;
Index: 2.6-mm/mm/page-writeback.c
===================================================================
--- 2.6-mm.orig/mm/page-writeback.c 2006-06-24 01:39:29.000000000 +0200
+++ 2.6-mm/mm/page-writeback.c 2006-06-24 01:40:20.000000000 +0200
@@ -29,6 +29,7 @@
#include <linux/sysctl.h>
#include <linux/cpu.h>
#include <linux/syscalls.h>
+#include <linux/rmap.h>

/*
* The maximum number of pages to writeout in a single bdflush/kupdate
@@ -566,7 +567,7 @@ int do_writepages(struct address_space *
return 0;
wbc->for_writepages = 1;
if (mapping->a_ops->writepages)
- ret = mapping->a_ops->writepages(mapping, wbc);
+ ret = mapping->a_ops->writepages(mapping, wbc);
else
ret = generic_writepages(mapping, wbc);
wbc->for_writepages = 0;
@@ -728,8 +729,14 @@ int test_clear_page_dirty(struct page *p
page_index(page),
PAGECACHE_TAG_DIRTY);
write_unlock_irqrestore(&mapping->tree_lock, flags);
- if (mapping_cap_account_dirty(mapping))
+ /*
+ * We can continue to use `mapping' here because the
+ * page is locked, which pins the address_space
+ */
+ if (mapping_cap_account_dirty(mapping)) {
+ page_mkclean(page);
dec_page_state(nr_dirty);
+ }
return 1;
}
write_unlock_irqrestore(&mapping->tree_lock, flags);
@@ -759,8 +766,10 @@ int clear_page_dirty_for_io(struct page

if (mapping) {
if (TestClearPageDirty(page)) {
- if (mapping_cap_account_dirty(mapping))
+ if (mapping_cap_account_dirty(mapping)) {
+ page_mkclean(page);
dec_page_state(nr_dirty);
+ }
return 1;
}
return 0;
Index: 2.6-mm/mm/rmap.c
===================================================================
--- 2.6-mm.orig/mm/rmap.c 2006-06-24 01:39:29.000000000 +0200
+++ 2.6-mm/mm/rmap.c 2006-06-24 01:40:20.000000000 +0200
@@ -53,6 +53,7 @@
#include <linux/rmap.h>
#include <linux/rcupdate.h>
#include <linux/module.h>
+#include <linux/backing-dev.h>

#include <asm/tlbflush.h>

@@ -434,6 +435,75 @@ int page_referenced(struct page *page, i
return referenced;
}

+static int page_mkclean_one(struct page *page, struct vm_area_struct *vma, int protect)
+{
+ struct mm_struct *mm = vma->vm_mm;
+ unsigned long address;
+ pte_t *pte, entry;
+ spinlock_t *ptl;
+ int ret = 0;
+
+ address = vma_address(page, vma);
+ if (address == -EFAULT)
+ goto out;
+
+ pte = page_check_address(page, mm, address, &ptl);
+ if (!pte)
+ goto out;
+
+ if (!(pte_dirty(*pte) || (protect && pte_write(*pte))))
+ goto unlock;
+
+ entry = ptep_get_and_clear(mm, address, pte);
+ entry = pte_mkclean(entry);
+ if (protect)
+ entry = pte_wrprotect(entry);
+ ptep_establish(vma, address, pte, entry);
+ lazy_mmu_prot_update(entry);
+ ret = 1;
+
+unlock:
+ pte_unmap_unlock(pte, ptl);
+out:
+ return ret;
+}
+
+static int page_mkclean_file(struct address_space *mapping, struct page *page)
+{
+ pgoff_t pgoff = page->index << (PAGE_CACHE_SHIFT - PAGE_SHIFT);
+ struct vm_area_struct *vma;
+ struct prio_tree_iter iter;
+ int ret = 0;
+
+ BUG_ON(PageAnon(page));
+
+ spin_lock(&mapping->i_mmap_lock);
+ vma_prio_tree_foreach(vma, &iter, &mapping->i_mmap, pgoff, pgoff) {
+ int protect = mapping_cap_account_dirty(mapping) &&
+ ((vma->vm_flags & (VM_WRITE|VM_SHARED)) ==
+ (VM_WRITE|VM_SHARED));
+ BUG_ON(vma->vm_flags & (VM_PFNMAP|VM_INSERTPAGE));
+ ret += page_mkclean_one(page, vma, protect);
+ }
+ spin_unlock(&mapping->i_mmap_lock);
+ return ret;
+}
+
+int page_mkclean(struct page *page)
+{
+ int ret = 0;
+
+ BUG_ON(!PageLocked(page));
+
+ if (page_mapped(page)) {
+ struct address_space *mapping = page_mapping(page);
+ if (mapping)
+ ret = page_mkclean_file(mapping, page);
+ }
+
+ return ret;
+}
+
/**
* page_set_anon_rmap - setup new anonymous rmap
* @page: the page to add the mapping to
Index: 2.6-mm/include/linux/rmap.h
===================================================================
--- 2.6-mm.orig/include/linux/rmap.h 2006-06-24 01:39:29.000000000 +0200
+++ 2.6-mm/include/linux/rmap.h 2006-06-24 01:40:20.000000000 +0200
@@ -103,6 +103,14 @@ pte_t *page_check_address(struct page *,
*/
unsigned long page_address_in_vma(struct page *, struct vm_area_struct *);

+/*
+ * Cleans the PTEs of shared mappings.
+ * (and since clean PTEs should also be readonly, write protects them too)
+ *
+ * returns the number of cleaned PTEs.
+ */
+int page_mkclean(struct page *);
+
#else /* !CONFIG_MMU */

#define anon_vma_init() do {} while (0)
Index: 2.6-mm/fs/buffer.c
===================================================================
--- 2.6-mm.orig/fs/buffer.c 2006-06-24 01:39:29.000000000 +0200
+++ 2.6-mm/fs/buffer.c 2006-06-24 01:40:20.000000000 +0200
@@ -2983,6 +2983,7 @@ int try_to_free_buffers(struct page *pag

spin_lock(&mapping->private_lock);
ret = drop_buffers(page, &buffers_to_free);
+ spin_unlock(&mapping->private_lock);
if (ret) {
/*
* If the filesystem writes its buffers by hand (eg ext3)
@@ -2994,7 +2995,6 @@ int try_to_free_buffers(struct page *pag
*/
clear_page_dirty(page);
}
- spin_unlock(&mapping->private_lock);
out:
if (buffers_to_free) {
struct buffer_head *bh = buffers_to_free;
Index: 2.6-mm/include/linux/mm.h
===================================================================
--- 2.6-mm.orig/include/linux/mm.h 2006-06-24 01:39:29.000000000 +0200
+++ 2.6-mm/include/linux/mm.h 2006-06-24 01:40:20.000000000 +0200
@@ -15,6 +15,7 @@
#include <linux/fs.h>
#include <linux/mutex.h>
#include <linux/debug_locks.h>
+#include <linux/backing-dev.h>

struct mempolicy;
struct anon_vma;
@@ -847,6 +848,39 @@ struct shrinker {
extern struct shrinker *set_shrinker(int, shrinker_t);
extern void remove_shrinker(struct shrinker *shrinker);

+/*
+ * Some shared mappigns will want the pages marked read-only
+ * to track write events. If so, we'll downgrade vm_page_prot
+ * to the private version (using protection_map[] without the
+ * VM_SHARED bit).
+ */
+static inline int vma_wants_writenotify(struct vm_area_struct *vma)
+{
+ unsigned int vm_flags = vma->vm_flags;
+
+ /* If it was private or non-writable, the write bit is already clear */
+ if ((vm_flags & (VM_WRITE|VM_SHARED)) != ((VM_WRITE|VM_SHARED)))
+ return 0;
+
+ /* The backer wishes to know when pages are first written to? */
+ if (vma->vm_ops && vma->vm_ops->page_mkwrite)
+ return 1;
+
+ /* The open routine did something to the protections already? */
+ if (pgprot_val(vma->vm_page_prot) !=
+ pgprot_val(protection_map[vm_flags &
+ (VM_READ|VM_WRITE|VM_EXEC|VM_SHARED)]))
+ return 0;
+
+ /* Specialty mapping? */
+ if (vm_flags & (VM_PFNMAP|VM_INSERTPAGE))
+ return 0;
+
+ /* Can the mapping track the dirty pages? */
+ return vma->vm_file && vma->vm_file->f_mapping &&
+ mapping_cap_account_dirty(vma->vm_file->f_mapping);
+}
+
extern pte_t *FASTCALL(get_locked_pte(struct mm_struct *mm, unsigned long addr, spinlock_t **ptl));

int __pud_alloc(struct mm_struct *mm, pgd_t *pgd, unsigned long address);