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[PATCH] mm: tracking shared dirty pages

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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>
This commit is contained in:
Peter Zijlstra
2006-09-25 23:30:57 -07:00
committed by Linus Torvalds
parent 725d704eca
commit d08b3851da
8 changed files with 162 additions and 30 deletions
Download Patch File Download Diff File Expand all files Collapse all files

29
mm/memory.c
View File

@ -1458,14 +1458,19 @@ static int do_wp_page(struct mm_struct *mm, struct vm_area_struct *vma,
{
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 *mm, struct vm_area_struct *vma,
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 *mm, struct vm_area_struct *vma,
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);