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

    From: Peter Zijlstra <a.p.zijlstra@chello.nl>

    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 -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.

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

    fs/buffer.c | 2 -
    include/linux/mm.h | 6 ++++
    include/linux/rmap.h | 8 ++++++
    mm/memory.c | 29 ++++++++++++++++++---
    mm/mmap.c | 34 +++++++++++++++++++++----
    mm/mprotect.c | 7 ++++-
    mm/page-writeback.c | 9 ++++++
    mm/rmap.c | 68 +++++++++++++++++++++++++++++++++++++++++++++++++++
    8 files changed, 149 insertions(+), 14 deletions(-)

    Index: 2.6-mm/include/linux/mm.h
    ===================================================================
    --- 2.6-mm.orig/include/linux/mm.h 2006-06-14 10:29:04.000000000 +0200
    +++ 2.6-mm/include/linux/mm.h 2006-06-19 14:45:18.000000000 +0200
    @@ -182,6 +182,12 @@ 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: 2.6-mm/mm/memory.c
    ===================================================================
    --- 2.6-mm.orig/mm/memory.c 2006-06-14 10:29:06.000000000 +0200
    +++ 2.6-mm/mm/memory.c 2006-06-19 16:20:06.000000000 +0200
    @@ -938,6 +938,12 @@ 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);
    @@ -1458,13 +1464,14 @@ 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)) {
    + if (unlikely(is_shared_writable(vma->vm_flags))) {
    if (vma->vm_ops && vma->vm_ops->page_mkwrite) {
    /*
    * Notify the address space that the page is about to
    @@ -1493,13 +1500,12 @@ 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) {
    @@ -1565,6 +1571,10 @@ 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)
    @@ -2097,6 +2107,7 @@ 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);
    @@ -2191,6 +2202,10 @@ 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. */
    @@ -2203,6 +2218,10 @@ 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 2006-06-14 10:29:06.000000000 +0200
    +++ 2.6-mm/mm/mmap.c 2006-06-19 15:41:53.000000000 +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))
    @@ -1084,18 +1086,13 @@ 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)
    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 +1110,31 @@ 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_ops && vma->vm_ops->page_mkwrite))
    + 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;
    Index: 2.6-mm/mm/mprotect.c
    ===================================================================
    --- 2.6-mm.orig/mm/mprotect.c 2006-06-14 10:29:06.000000000 +0200
    +++ 2.6-mm/mm/mprotect.c 2006-06-19 16:19:42.000000000 +0200
    @@ -21,6 +21,7 @@
    #include <linux/syscalls.h>
    #include <linux/swap.h>
    #include <linux/swapops.h>
    +#include <linux/backing-dev.h>
    #include <asm/uaccess.h>
    #include <asm/pgtable.h>
    #include <asm/cacheflush.h>
    @@ -124,6 +125,7 @@ mprotect_fixup(struct vm_area_struct *vm
    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;
    @@ -179,7 +181,10 @@ 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)
    + if (is_shared_writable(newflags) && vma->vm_file)
    + mapping = vma->vm_file->f_mapping;
    + if ((mapping && mapping_cap_account_dirty(mapping)) ||
    + (vma->vm_ops && vma->vm_ops->page_mkwrite))
    mask &= ~VM_SHARED;

    newprot = protection_map[newflags & mask];
    Index: 2.6-mm/mm/page-writeback.c
    ===================================================================
    --- 2.6-mm.orig/mm/page-writeback.c 2006-06-14 10:29:07.000000000 +0200
    +++ 2.6-mm/mm/page-writeback.c 2006-06-19 16:19:42.000000000 +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
    @@ -540,7 +541,7 @@ 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;
    @@ -704,6 +705,11 @@ int test_clear_page_dirty(struct page *p
    if (mapping_cap_account_dirty(mapping))
    __dec_zone_page_state(page, NR_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);
    return 1;
    }
    write_unlock_irqrestore(&mapping->tree_lock, flags);
    @@ -733,6 +739,7 @@ int clear_page_dirty_for_io(struct page

    if (mapping) {
    if (TestClearPageDirty(page)) {
    + page_mkclean(page);
    if (mapping_cap_account_dirty(mapping))
    dec_zone_page_state(page, NR_DIRTY);
    return 1;
    Index: 2.6-mm/mm/rmap.c
    ===================================================================
    --- 2.6-mm.orig/mm/rmap.c 2006-06-14 10:29:07.000000000 +0200
    +++ 2.6-mm/mm/rmap.c 2006-06-19 14:45:18.000000000 +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 +435,73 @@ 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: 2.6-mm/include/linux/rmap.h
    ===================================================================
    --- 2.6-mm.orig/include/linux/rmap.h 2006-06-14 10:29:04.000000000 +0200
    +++ 2.6-mm/include/linux/rmap.h 2006-06-19 14:45:18.000000000 +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 2006-06-14 10:28:52.000000000 +0200
    +++ 2.6-mm/fs/buffer.c 2006-06-19 14:45:18.000000000 +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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