Re: [rfc][patch] mm: dirty page accounting race fix

Peter Zijlstra
2008-08-14T10:27:08Z

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;