Messages in this thread | | | Subject | Re: [rfc][patch] mm: dirty page accounting race fix | From | Peter Zijlstra <> | Date | Thu, 14 Aug 2008 12:27:08 +0200 |
| |
On Thu, 2008-08-14 at 11:45 +0200, Nick Piggin wrote: > Hi, > > Wonder if you would be kind enough to verify this? It solves my issues (well > still running after several hours wheras it previously would lock up in a > few seconds). > > Thanks, > Nick
s/synch/sync/ ?
Indeed, that does look like a hole, and the proposed fix seems sane.
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
> --- > There is a race with dirty page accounting where a page may not properly > be accounted for. > > clear_page_dirty_for_io() calls page_mkclean; then TestClearPageDirty. > > page_mkclean walks the rmaps for that page, and for each one it cleans and > write protects the pte if it was dirty. It uses page_check_address to find the > pte. That function has a shortcut to avoid the ptl if the pte is not > present. Unfortunately, the pte can be switched to not-present then back to > present by other code while holding the page table lock -- this should not > be a signal for page_mkclean to ignore that pte, because it may be dirty. > > For example, do_wp_page calls ptep_clear_flush_notify before marking the > pte writable and dirty. do_wp_page does subsequently set the page dirty, but > that can happen before clear_page_dirty_for_io() calls TestClearPageDirty. > There may also be other code which does similar things, and/or architectures > which implement other pte manipulation functions using a clear-set sequence, > I haven't looked thoroughly. But it should be fixed. > > The consequence of the bug is loss of data integrity (msync) of dirty page > accounting accuracy. XIP's __xip_unmap is probably also unreliable, which > can lead to data corruption. > > Fix this by always taking ptl to check the pte in page_check_address. > > It's possible to retain this optimization for page_referenced and > try_to_unmap. > > Signed-off-by: Nick Piggin <npiggin@suse.de> > --- > Index: linux-2.6/include/linux/rmap.h > =================================================================== > --- linux-2.6.orig/include/linux/rmap.h > +++ linux-2.6/include/linux/rmap.h > @@ -102,7 +102,7 @@ int try_to_unmap(struct page *, int igno > * Called from mm/filemap_xip.c to unmap empty zero page > */ > pte_t *page_check_address(struct page *, struct mm_struct *, > - unsigned long, spinlock_t **); > + unsigned long, spinlock_t **, int); > > /* > * Used by swapoff to help locate where page is expected in vma. > Index: linux-2.6/mm/filemap_xip.c > =================================================================== > --- linux-2.6.orig/mm/filemap_xip.c > +++ linux-2.6/mm/filemap_xip.c > @@ -185,7 +185,7 @@ __xip_unmap (struct address_space * mapp > address = vma->vm_start + > ((pgoff - vma->vm_pgoff) << PAGE_SHIFT); > BUG_ON(address < vma->vm_start || address >= vma->vm_end); > - pte = page_check_address(page, mm, address, &ptl); > + pte = page_check_address(page, mm, address, &ptl, 1); > if (pte) { > /* Nuke the page table entry. */ > flush_cache_page(vma, address, pte_pfn(*pte)); > Index: linux-2.6/mm/rmap.c > =================================================================== > --- linux-2.6.orig/mm/rmap.c > +++ linux-2.6/mm/rmap.c > @@ -224,10 +224,14 @@ unsigned long page_address_in_vma(struct > /* > * Check that @page is mapped at @address into @mm. > * > + * If @synch is false, page_check_address may perform a racy check to avoid > + * the page table lock when the pte is not present (helpful when reclaiming > + * highly shared pages). > + * > * On success returns with pte mapped and locked. > */ > pte_t *page_check_address(struct page *page, struct mm_struct *mm, > - unsigned long address, spinlock_t **ptlp) > + unsigned long address, spinlock_t **ptlp, int synch) > { > pgd_t *pgd; > pud_t *pud; > @@ -249,7 +253,7 @@ pte_t *page_check_address(struct page *p > > pte = pte_offset_map(pmd, address); > /* Make a quick check before getting the lock */ > - if (!pte_present(*pte)) { > + if (!synch && !pte_present(*pte)) { > pte_unmap(pte); > return NULL; > } > @@ -281,7 +285,7 @@ static int page_referenced_one(struct pa > if (address == -EFAULT) > goto out; > > - pte = page_check_address(page, mm, address, &ptl); > + pte = page_check_address(page, mm, address, &ptl, 0); > if (!pte) > goto out; > > @@ -450,7 +454,7 @@ static int page_mkclean_one(struct page > if (address == -EFAULT) > goto out; > > - pte = page_check_address(page, mm, address, &ptl); > + pte = page_check_address(page, mm, address, &ptl, 1); > if (!pte) > goto out; > > @@ -697,7 +701,7 @@ static int try_to_unmap_one(struct page > if (address == -EFAULT) > goto out; > > - pte = page_check_address(page, mm, address, &ptl); > + pte = page_check_address(page, mm, address, &ptl, 0); > if (!pte) > goto out; >
| |