Re: [patch] mm: dirty page tracking race fix

[Nick Piggin]

On Tue, Aug 19, 2008 at 02:11:55AM -0700, Andrew Morton wrote:
> On Mon, 18 Aug 2008 07:38:21 +0200 Nick Piggin <npiggin@xxxxxxx> wrote:
> 
> > 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, powerpc64's set_pte_at will clear a previously present pte before
> > setting it to the desired value. There may also be other code in core mm or
> > in arch which do similar things.
> > 
> > The consequence of the bug is loss of data integrity due to msync, and loss
> > of dirty page accounting accuracy. XIP's __xip_unmap could easily also be
> > unreliable (depending on the exact XIP locking scheme), which can lead to data
> > corruption.
> > 
> > Fix this by having an option to always take ptl to check the pte in
> > page_check_address.
> > 
> > It's possible to retain this optimization for page_referenced and
> > try_to_unmap.
> 
> Is it also possible to retain it for
> 
> /**
>  * page_mapped_in_vma - check whether a page is really mapped in a VMA
>  * @page: the page to test
>  * @vma: the VMA to test
>  *
>  * Returns 1 if the page is mapped into the page tables of the VMA, 0
>  * if the page is not mapped into the page tables of this VMA.  Only
>  * valid for normal file or anonymous VMAs.
>  */
> static int page_mapped_in_vma(struct page *page, struct vm_area_struct *vma)
> {
> 	unsigned long address;
> 	pte_t *pte;
> 	spinlock_t *ptl;
> 
> 	address = vma_address(page, vma);
> 	if (address == -EFAULT)		/* out of vma range */
> 		return 0;
> 	pte = page_check_address(page, vma->vm_mm, address, &ptl);
> 	if (!pte)			/* the page is not in this mm */
> 		return 0;
> 	pte_unmap_unlock(pte, ptl);
> 
> 	return 1;
> }
> 
> ?

No. While callers of page_mapped_in_vma might be OK with it, the function
comments themselves indicate we can't take the shortcut.

It was possible the retain the optimization for page_referenced and
try_to_unmap because it is OK if those guys fail in rare cases. Page
reclaim will do a final non-racy check of refcounts.
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