Re: [patch 8/6] mm: fix cpdfio vs fault race

[Andrew Morton]

(cc's reestablished yet again)

On Wed, 7 Mar 2007 12:04:29 +0100 Nick Piggin <npiggin@xxxxxxx> wrote:

> OK, this is how we can plug that hole, leveraging my
> previous patches to lock page over do_no_page.
> 
> I'm pretty sure the PageLocked invariant is correct.
> 
> 
> --
> Fix msync data loss and (less importantly) dirty page accounting inaccuracies
> due to the race remaining in clear_page_dirty_for_io().
> 
> The deleted comment explains what the race was, and the added comments
> explain how it is fixed.
> 
> Signed-off-by: Nick Piggin <npiggin@xxxxxxx>
> 
> Index: linux-2.6/mm/memory.c
> ===================================================================
> --- linux-2.6.orig/mm/memory.c
> +++ linux-2.6/mm/memory.c
> @@ -1676,6 +1676,17 @@ gotten:
>  unlock:
>  	pte_unmap_unlock(page_table, ptl);
>  	if (dirty_page) {
> +		/*
> +		 * Yes, Virginia, this is actually required to prevent a race
> +		 * with clear_page_dirty_for_io() from clearing the page dirty
> +		 * bit after it clear all dirty ptes, but before a racing
> +		 * do_wp_page installs a dirty pte.
> +		 *
> +		 * do_fault is protected similarly by holding the page lock
> +		 * after the dirty pte is installed.
> +		 */
> +		lock_page(dirty_page);
> +		unlock_page(dirty_page);
>  		set_page_dirty_balance(dirty_page);
>  		put_page(dirty_page);

Yes, I think that'll plug it.  A wait_on_page_locked() should suffice.

But does this have any dependency on the lock-page-over-do_no_page patches?


>  	}
> Index: linux-2.6/mm/page-writeback.c
> ===================================================================
> --- linux-2.6.orig/mm/page-writeback.c
> +++ linux-2.6/mm/page-writeback.c
> @@ -903,6 +903,8 @@ int clear_page_dirty_for_io(struct page 
>  {
>  	struct address_space *mapping = page_mapping(page);
>  
> +	BUG_ON(!PageLocked(page));
> +
>  	if (mapping && mapping_cap_account_dirty(mapping)) {
>  		/*
>  		 * Yes, Virginia, this is indeed insane.
> @@ -928,14 +930,19 @@ int clear_page_dirty_for_io(struct page 
>  		 * We basically use the page "master dirty bit"
>  		 * as a serialization point for all the different
>  		 * threads doing their things.
> -		 *
> -		 * FIXME! We still have a race here: if somebody
> -		 * adds the page back to the page tables in
> -		 * between the "page_mkclean()" and the "TestClearPageDirty()",
> -		 * we might have it mapped without the dirty bit set.
>  		 */
>  		if (page_mkclean(page))
>  			set_page_dirty(page);
> +		/*
> +		 * We carefully synchronise fault handlers against
> +		 * installing a dirty pte and marking the page dirty
> +		 * at this point. We do this by having them hold the
> +		 * page lock at some point after installing their
> +		 * pte, but before marking the page dirty.
> +		 * Pages are always locked coming in here, so we get
> +		 * the desired exclusion. See mm/memory.c:do_wp_page()
> +		 * for more comments.
> +		 */
>  		if (TestClearPageDirty(page)) {
>  			dec_zone_page_state(page, NR_FILE_DIRTY);
>  			return 1;
-
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