Re: [patch 1/3] mm: protect set_page_dirty() from ongoing truncation

[Jan Kara]

On Fri 05-12-14 09:52:44, Johannes Weiner wrote:
> Tejun, while reviewing the code, spotted the following race condition
> between the dirtying and truncation of a page:
> 
> __set_page_dirty_nobuffers()       __delete_from_page_cache()
>   if (TestSetPageDirty(page))
>                                      page->mapping = NULL
> 				     if (PageDirty())
> 				       dec_zone_page_state(page, NR_FILE_DIRTY);
> 				       dec_bdi_stat(mapping->backing_dev_info, BDI_RECLAIMABLE);
>     if (page->mapping)
>       account_page_dirtied(page)
>         __inc_zone_page_state(page, NR_FILE_DIRTY);
> 	__inc_bdi_stat(mapping->backing_dev_info, BDI_RECLAIMABLE);
> 
> which results in an imbalance of NR_FILE_DIRTY and BDI_RECLAIMABLE.
> 
> Dirtiers usually lock out truncation, either by holding the page lock
> directly, or in case of zap_pte_range(), by pinning the mapcount with
> the page table lock held.  The notable exception to this rule, though,
> is do_wp_page(), for which this race exists.  However, do_wp_page()
> already waits for a locked page to unlock before setting the dirty
> bit, in order to prevent a race where clear_page_dirty() misses the
> page bit in the presence of dirty ptes.  Upgrade that wait to a fully
> locked set_page_dirty() to also cover the situation explained above.
> 
> Afterwards, the code in set_page_dirty() dealing with a truncation
> race is no longer needed.  Remove it.
> 
> Reported-by: Tejun Heo <tj@xxxxxxxxxx>
> Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx>
> Cc: <stable@xxxxxxxxxxxxxxx>
> Acked-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
  Looks good. You can add:
Reviewed-by: Jan Kara <jack@xxxxxxx>

								Honza

> ---
>  include/linux/writeback.h |  1 -
>  mm/memory.c               | 27 +++++++++++++++++----------
>  mm/page-writeback.c       | 43 ++++++++++++-------------------------------
>  3 files changed, 29 insertions(+), 42 deletions(-)
> 
> diff --git a/include/linux/writeback.h b/include/linux/writeback.h
> index a219be961c0a..00048339c23e 100644
> --- a/include/linux/writeback.h
> +++ b/include/linux/writeback.h
> @@ -177,7 +177,6 @@ int write_cache_pages(struct address_space *mapping,
>  		      struct writeback_control *wbc, writepage_t writepage,
>  		      void *data);
>  int do_writepages(struct address_space *mapping, struct writeback_control *wbc);
> -void set_page_dirty_balance(struct page *page);
>  void writeback_set_ratelimit(void);
>  void tag_pages_for_writeback(struct address_space *mapping,
>  			     pgoff_t start, pgoff_t end);
> diff --git a/mm/memory.c b/mm/memory.c
> index 3e503831e042..72d998eb0438 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -2150,17 +2150,24 @@ reuse:
>  		if (!dirty_page)
>  			return ret;
>  
> -		/*
> -		 * 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_shared_fault is protected similarly.
> -		 */
>  		if (!page_mkwrite) {
> -			wait_on_page_locked(dirty_page);
> -			set_page_dirty_balance(dirty_page);
> +			struct address_space *mapping;
> +			int dirtied;
> +
> +			lock_page(dirty_page);
> +			dirtied = set_page_dirty(dirty_page);
> +			VM_BUG_ON_PAGE(PageAnon(dirty_page), dirty_page);
> +			mapping = dirty_page->mapping;
> +			unlock_page(dirty_page);
> +
> +			if (dirtied && mapping) {
> +				/*
> +				 * Some device drivers do not set page.mapping
> +				 * but still dirty their pages
> +				 */
> +				balance_dirty_pages_ratelimited(mapping);
> +			}
> +
>  			/* file_update_time outside page_lock */
>  			if (vma->vm_file)
>  				file_update_time(vma->vm_file);
> diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> index 19ceae87522d..437174a2aaa3 100644
> --- a/mm/page-writeback.c
> +++ b/mm/page-writeback.c
> @@ -1541,16 +1541,6 @@ pause:
>  		bdi_start_background_writeback(bdi);
>  }
>  
> -void set_page_dirty_balance(struct page *page)
> -{
> -	if (set_page_dirty(page)) {
> -		struct address_space *mapping = page_mapping(page);
> -
> -		if (mapping)
> -			balance_dirty_pages_ratelimited(mapping);
> -	}
> -}
> -
>  static DEFINE_PER_CPU(int, bdp_ratelimits);
>  
>  /*
> @@ -2123,32 +2113,25 @@ EXPORT_SYMBOL(account_page_dirtied);
>   * page dirty in that case, but not all the buffers.  This is a "bottom-up"
>   * dirtying, whereas __set_page_dirty_buffers() is a "top-down" dirtying.
>   *
> - * Most callers have locked the page, which pins the address_space in memory.
> - * But zap_pte_range() does not lock the page, however in that case the
> - * mapping is pinned by the vma's ->vm_file reference.
> - *
> - * We take care to handle the case where the page was truncated from the
> - * mapping by re-checking page_mapping() inside tree_lock.
> + * The caller must ensure this doesn't race with truncation.  Most will simply
> + * hold the page lock, but e.g. zap_pte_range() calls with the page mapped and
> + * the pte lock held, which also locks out truncation.
>   */
>  int __set_page_dirty_nobuffers(struct page *page)
>  {
>  	if (!TestSetPageDirty(page)) {
>  		struct address_space *mapping = page_mapping(page);
> -		struct address_space *mapping2;
>  		unsigned long flags;
>  
>  		if (!mapping)
>  			return 1;
>  
>  		spin_lock_irqsave(&mapping->tree_lock, flags);
> -		mapping2 = page_mapping(page);
> -		if (mapping2) { /* Race with truncate? */
> -			BUG_ON(mapping2 != mapping);
> -			WARN_ON_ONCE(!PagePrivate(page) && !PageUptodate(page));
> -			account_page_dirtied(page, mapping);
> -			radix_tree_tag_set(&mapping->page_tree,
> -				page_index(page), PAGECACHE_TAG_DIRTY);
> -		}
> +		BUG_ON(page_mapping(page) != mapping);
> +		WARN_ON_ONCE(!PagePrivate(page) && !PageUptodate(page));
> +		account_page_dirtied(page, mapping);
> +		radix_tree_tag_set(&mapping->page_tree, page_index(page),
> +				   PAGECACHE_TAG_DIRTY);
>  		spin_unlock_irqrestore(&mapping->tree_lock, flags);
>  		if (mapping->host) {
>  			/* !PageAnon && !swapper_space */
> @@ -2305,12 +2288,10 @@ int clear_page_dirty_for_io(struct page *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.
> +		 * at this point.  We do this by having them hold the
> +		 * page lock while dirtying the page, and pages are
> +		 * always locked coming in here, so we get the desired
> +		 * exclusion.
>  		 */
>  		if (TestClearPageDirty(page)) {
>  			dec_zone_page_state(page, NR_FILE_DIRTY);
> -- 
> 2.1.3
> 
-- 
Jan Kara <jack@xxxxxxx>
SUSE Labs, CR
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