[PATCH 1/6] mm: tracking shared dirty pages

From: Peter Zijlstra
Date: Tue Jun 13 2006 - 07:21:07 EST


From: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>

People expressed the need to track dirty pages in shared mappings.

Linus outlined the general idea of doing that through making clean
writable pages write-protected and taking the write fault.

This patch does exactly that, it makes pages in a shared writable
mapping write-protected. On write-fault the pages are marked dirty and
made writable. When the pages get synced with their backing store, the
write-protection is re-instated.

It survives a simple test and shows the dirty pages in /proc/vmstat.

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.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
---

fs/buffer.c | 2 -
include/linux/mm.h | 6 ++++
include/linux/rmap.h | 8 ++++++
mm/memory.c | 56 ++++++++++++++++++++++++++++++++----------
mm/mmap.c | 33 +++++++++++++++++++++++-
mm/mprotect.c | 14 +++++++++-
mm/page-writeback.c | 9 ++++++
mm/rmap.c | 68 +++++++++++++++++++++++++++++++++++++++++++++++++++
8 files changed, 177 insertions(+), 19 deletions(-)

Index: linux-2.6/include/linux/mm.h
===================================================================
--- linux-2.6.orig/include/linux/mm.h 2006-06-12 07:01:34.000000000 +0200
+++ linux-2.6/include/linux/mm.h 2006-06-12 13:09:55.000000000 +0200
@@ -183,6 +183,12 @@ extern unsigned int kobjsize(const void
#define VM_SequentialReadHint(v) ((v)->vm_flags & VM_SEQ_READ)
#define VM_RandomReadHint(v) ((v)->vm_flags & VM_RAND_READ)

+static inline int is_shared_writable(unsigned int flags)
+{
+ return (flags & (VM_SHARED|VM_WRITE|VM_PFNMAP)) ==
+ (VM_SHARED|VM_WRITE);
+}
+
/*
* mapping from the currently active vm_flags protection bits (the
* low four bits) to a page protection mask..
Index: linux-2.6/mm/memory.c
===================================================================
--- linux-2.6.orig/mm/memory.c 2006-06-12 07:01:34.000000000 +0200
+++ linux-2.6/mm/memory.c 2006-06-13 11:17:51.000000000 +0200
@@ -925,6 +925,12 @@ struct page *follow_page(struct vm_area_
pte = *ptep;
if (!pte_present(pte))
goto unlock;
+ /*
+ * This is not fully correct in the light of trapping write faults
+ * for writable shared mappings. However since we're going to mark
+ * the page dirty anyway some few lines downward, we might as well
+ * take the write fault now.
+ */
if ((flags & FOLL_WRITE) && !pte_write(pte))
goto unlock;
page = vm_normal_page(vma, address, pte);
@@ -1445,25 +1451,36 @@ static int do_wp_page(struct mm_struct *
{
struct page *old_page, *new_page;
pte_t entry;
- int 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 (PageAnon(old_page) && !TestSetPageLocked(old_page)) {
- int reuse = can_share_swap_page(old_page);
+ /* get_user_pages(.write:1, .force:1)
+ * __handle_mm_fault()
+ *
+ * Makes COW happen for readonly shared mappings too.
+ */
+ if (unlikely(is_shared_writable(vma->vm_flags))) {
+ reuse = 1;
+ dirty_page = old_page;
+ get_page(dirty_page);
+ } else if (PageAnon(old_page) && !TestSetPageLocked(old_page)) {
+ reuse = can_share_swap_page(old_page);
unlock_page(old_page);
- if (reuse) {
- flush_cache_page(vma, address, pte_pfn(orig_pte));
- entry = pte_mkyoung(orig_pte);
- entry = maybe_mkwrite(pte_mkdirty(entry), vma);
- ptep_set_access_flags(vma, address, page_table, entry, 1);
- update_mmu_cache(vma, address, entry);
- lazy_mmu_prot_update(entry);
- ret |= VM_FAULT_WRITE;
- goto unlock;
- }
+ }
+
+ if (reuse) {
+ flush_cache_page(vma, address, pte_pfn(orig_pte));
+ entry = pte_mkyoung(orig_pte);
+ entry = maybe_mkwrite(pte_mkdirty(entry), vma);
+ ptep_set_access_flags(vma, address, page_table, entry, 1);
+ update_mmu_cache(vma, address, entry);
+ lazy_mmu_prot_update(entry);
+ ret |= VM_FAULT_WRITE;
+ goto unlock;
}

/*
@@ -1518,6 +1535,10 @@ 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)
@@ -2046,6 +2067,7 @@ 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);
@@ -2127,6 +2149,10 @@ 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. */
@@ -2139,6 +2165,10 @@ 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: linux-2.6/mm/mmap.c
===================================================================
--- linux-2.6.orig/mm/mmap.c 2006-06-12 07:01:34.000000000 +0200
+++ linux-2.6/mm/mmap.c 2006-06-12 14:40:50.000000000 +0200
@@ -25,6 +25,7 @@
#include <linux/mount.h>
#include <linux/mempolicy.h>
#include <linux/rmap.h>
+#include <linux/backing-dev.h>

#include <asm/uaccess.h>
#include <asm/cacheflush.h>
@@ -888,6 +889,7 @@ unsigned long do_mmap_pgoff(struct file
struct rb_node ** rb_link, * rb_parent;
int accountable = 1;
unsigned long charged = 0, reqprot = prot;
+ struct address_space *mapping = NULL;

if (file) {
if (is_file_hugepages(file))
@@ -1065,7 +1067,8 @@ munmap_back:
vma->vm_start = addr;
vma->vm_end = addr + len;
vma->vm_flags = vm_flags;
- vma->vm_page_prot = protection_map[vm_flags & 0x0f];
+ vma->vm_page_prot = protection_map[vm_flags &
+ (VM_READ|VM_WRITE|VM_EXEC|VM_SHARED)];
vma->vm_pgoff = pgoff;

if (file) {
@@ -1083,6 +1086,7 @@ munmap_back:
error = file->f_op->mmap(file, vma);
if (error)
goto unmap_and_free_vma;
+
} else if (vm_flags & VM_SHARED) {
error = shmem_zero_setup(vma);
if (error)
@@ -1106,6 +1110,30 @@ munmap_back:
pgoff = vma->vm_pgoff;
vm_flags = vma->vm_flags;

+ /*
+ * Tracking of dirty pages for shared writable mappings. Do this by
+ * write protecting writable pages, and mark dirty in the write fault.
+ *
+ * Modify vma->vm_page_prot (the default protection for new pages)
+ * to this effect.
+ *
+ * Cannot do before because the condition depends on:
+ * - backing_dev_info having the right capabilities
+ * (set by f_op->open())
+ * - vma->vm_flags being fully set
+ * (finished in f_op->mmap(), which could call remap_pfn_range())
+ *
+ * Also, cannot reset vma->vm_page_prot from vma->vm_flags because
+ * f_op->mmap() can modify it.
+ */
+ if (is_shared_writable(vm_flags) && vma->vm_file)
+ mapping = vma->vm_file->f_mapping;
+ if (mapping && mapping_cap_account_dirty(mapping))
+ vma->vm_page_prot =
+ __pgprot(pte_val
+ (pte_wrprotect
+ (__pte(pgprot_val(vma->vm_page_prot)))));
+
if (!file || !vma_merge(mm, prev, addr, vma->vm_end,
vma->vm_flags, NULL, file, pgoff, vma_policy(vma))) {
file = vma->vm_file;
@@ -1921,7 +1949,8 @@ unsigned long do_brk(unsigned long addr,
vma->vm_end = addr + len;
vma->vm_pgoff = pgoff;
vma->vm_flags = flags;
- vma->vm_page_prot = protection_map[flags & 0x0f];
+ vma->vm_page_prot = protection_map[flags &
+ (VM_READ|VM_WRITE|VM_EXEC|VM_SHARED)];
vma_link(mm, vma, prev, rb_link, rb_parent);
out:
mm->total_vm += len >> PAGE_SHIFT;
Index: linux-2.6/mm/mprotect.c
===================================================================
--- linux-2.6.orig/mm/mprotect.c 2006-06-12 07:01:34.000000000 +0200
+++ linux-2.6/mm/mprotect.c 2006-06-12 13:09:55.000000000 +0200
@@ -19,6 +19,7 @@
#include <linux/mempolicy.h>
#include <linux/personality.h>
#include <linux/syscalls.h>
+#include <linux/backing-dev.h>

#include <asm/uaccess.h>
#include <asm/pgtable.h>
@@ -106,6 +107,8 @@ 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;
+ struct address_space *mapping = NULL;
pgprot_t newprot;
pgoff_t pgoff;
int error;
@@ -132,8 +135,6 @@ mprotect_fixup(struct vm_area_struct *vm
}
}

- newprot = protection_map[newflags & 0xf];
-
/*
* First try to merge with previous and/or next vma.
*/
@@ -160,6 +161,15 @@ mprotect_fixup(struct vm_area_struct *vm
}

success:
+ /* Don't make the VMA automatically writable if it's shared. */
+ mask = VM_READ|VM_WRITE|VM_EXEC|VM_SHARED;
+ if (is_shared_writable(newflags) && vma->vm_file)
+ mapping = vma->vm_file->f_mapping;
+ if (mapping && mapping_cap_account_dirty(mapping))
+ mask &= ~VM_SHARED;
+
+ newprot = protection_map[newflags & mask];
+
/*
* vm_flags and vm_page_prot are protected by the mmap_sem
* held in write mode.
Index: linux-2.6/mm/page-writeback.c
===================================================================
--- linux-2.6.orig/mm/page-writeback.c 2006-06-12 07:01:34.000000000 +0200
+++ linux-2.6/mm/page-writeback.c 2006-06-13 11:17:40.000000000 +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
@@ -563,7 +564,7 @@ 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;
@@ -725,6 +726,11 @@ int test_clear_page_dirty(struct page *p
page_index(page),
PAGECACHE_TAG_DIRTY);
write_unlock_irqrestore(&mapping->tree_lock, flags);
+ /*
+ * We can continue to use `mapping' here because the
+ * page is locked, which pins the address_space
+ */
+ page_mkclean(page);
if (mapping_cap_account_dirty(mapping))
dec_page_state(nr_dirty);
return 1;
@@ -756,6 +762,7 @@ int clear_page_dirty_for_io(struct page

if (mapping) {
if (TestClearPageDirty(page)) {
+ page_mkclean(page);
if (mapping_cap_account_dirty(mapping))
dec_page_state(nr_dirty);
return 1;
Index: linux-2.6/mm/rmap.c
===================================================================
--- linux-2.6.orig/mm/rmap.c 2006-06-12 07:01:34.000000000 +0200
+++ linux-2.6/mm/rmap.c 2006-06-13 11:18:15.000000000 +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>

@@ -472,6 +473,73 @@ 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) &&
+ is_shared_writable(vma->vm_flags);
+ 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: linux-2.6/include/linux/rmap.h
===================================================================
--- linux-2.6.orig/include/linux/rmap.h 2006-06-12 07:01:34.000000000 +0200
+++ linux-2.6/include/linux/rmap.h 2006-06-12 13:09:55.000000000 +0200
@@ -105,6 +105,14 @@ 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: linux-2.6/fs/buffer.c
===================================================================
--- linux-2.6.orig/fs/buffer.c 2006-06-12 07:01:34.000000000 +0200
+++ linux-2.6/fs/buffer.c 2006-06-12 13:09:55.000000000 +0200
@@ -2985,6 +2985,7 @@ 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)
@@ -2996,7 +2997,6 @@ 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;
-
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