Re: [PATCH 2/2] mm: fix data corruption caused by lazyfree page

[Minchan Kim <minchan@kernel.org>]

On Thu, Sep 21, 2017 at 01:27:11PM -0700, Shaohua Li wrote:
> From: Shaohua Li <shli@fb.com>
> 
> MADV_FREE clears pte dirty bit and then marks the page lazyfree (clear
> SwapBacked). There is no lock to prevent the page is added to swap cache
> between these two steps by page reclaim. If page reclaim finds such
> page, it will simply add the page to swap cache without pageout the page
> to swap because the page is marked as clean. Next time, page fault will
> read data from the swap slot which doesn't have the original data, so we
> have a data corruption. To fix issue, we mark the page dirty and pageout
> the page.
> 
> However, we shouldn't dirty all pages which is clean and in swap cache.
> swapin page is swap cache and clean too. So we only dirty page which is
> added into swap cache in page reclaim, which shouldn't be swapin page.
> Normal anonymous pages should be dirty already.
> 
> Reported-and-tested-y: Artem Savkov <asavkov@redhat.com>
> Fix: 802a3a92ad7a(mm: reclaim MADV_FREE pages)
> Signed-off-by: Shaohua Li <shli@fb.com>
> Cc: Johannes Weiner <hannes@cmpxchg.org>
> Cc: Michal Hocko <mhocko@suse.com>
> Cc: Hillf Danton <hillf.zj@alibaba-inc.com>
> Cc: Minchan Kim <minchan@kernel.org>
> Cc: Hugh Dickins <hughd@google.com>
> Cc: Rik van Riel <riel@redhat.com>
> Cc: Mel Gorman <mgorman@techsingularity.net>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> ---
>  mm/vmscan.c | 12 ++++++++++++
>  1 file changed, 12 insertions(+)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index d811c81..820ee8d 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -980,6 +980,7 @@ static unsigned long shrink_page_list(struct list_head *page_list,
>  		int may_enter_fs;
>  		enum page_references references = PAGEREF_RECLAIM_CLEAN;
>  		bool dirty, writeback;
> +		bool new_swap_page = false;
>  
>  		cond_resched();
>  
> @@ -1165,6 +1166,7 @@ static unsigned long shrink_page_list(struct list_head *page_list,
>  
>  				/* Adding to swap updated mapping */
>  				mapping = page_mapping(page);
> +				new_swap_page = true;
>  			}
>  		} else if (unlikely(PageTransHuge(page))) {
>  			/* Split file THP */
> @@ -1185,6 +1187,16 @@ static unsigned long shrink_page_list(struct list_head *page_list,
>  				nr_unmap_fail++;
>  				goto activate_locked;
>  			}
> +
> +			/*
> +			 * MADV_FREE clear pte dirty bit, but not yet clear
> +			 * SwapBacked for a page. We can't directly free the
> +			 * page because we already set swap entry in pte. The
> +			 * check guarantees this is such page and not a clean
> +			 * swapin page
> +			 */
> +			if (!PageDirty(page) && new_swap_page)
> +				set_page_dirty(page);
>  		}
>  
>  		if (PageDirty(page)) {
> -- 
> 2.9.5
> 

Couldn't we simple roll back to the logic before MADV_FREE's birth?

diff --git a/mm/swap_state.c b/mm/swap_state.c
index 71ce2d1ccbf7..548c19b5f78e 100644
--- a/mm/swap_state.c
+++ b/mm/swap_state.c
@@ -231,7 +231,7 @@ int add_to_swap(struct page *page)
 	 * deadlock in the swap out path.
 	 */
 	/*
-	 * Add it to the swap cache.
+	 * Add it to the swap cache and mark it dirty
 	 */
 	err = add_to_swap_cache(page, entry,
 			__GFP_HIGH|__GFP_NOMEMALLOC|__GFP_NOWARN);
@@ -243,6 +243,7 @@ int add_to_swap(struct page *page)
 		 */
 		goto fail;
 
+	SetPageDirty(page);
 	return 1;
 
 fail:

To me, it would be more simple/readable rather than introducing
a new branch in complicated shrink_page_list.

And I don't see why we cannot merge [1/2] and [2/2].

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