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Date:   Thu, 19 Mar 2020 03:12:58 +0300
From:   "Kirill A. Shutemov" <kirill@shutemov.name>
To:     Yang Shi <yang.shi@linux.alibaba.com>
Cc:     kirill.shutemov@linux.intel.com, hughd@google.com,
        aarcange@redhat.com, akpm@linux-foundation.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] mm: khugepaged: fix potential page state corruption
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On Thu, Mar 19, 2020 at 07:19:42AM +0800, Yang Shi wrote:
> When khugepaged collapses anonymous pages, the base pages would be freed
> via pagevec or free_page_and_swap_cache().  But, the anonymous page may
> be added back to LRU, then it might result in the below race:
> 
> 	CPU A				CPU B
> khugepaged:
>   unlock page
>   putback_lru_page
>     add to lru
> 				page reclaim:
> 				  isolate this page
> 				  try_to_unmap
>   page_remove_rmap <-- corrupt _mapcount
> 
> It looks nothing would prevent the pages from isolating by reclaimer.

Hm. Why should it?

try_to_unmap() doesn't exclude parallel page unmapping. _mapcount is
protected by ptl. And this particular _mapcount pin is reachable for
reclaim as it's not part of usual page table tree. Basically
try_to_unmap() will never succeeds until we give up the _mapcount on
khugepaged side.

I don't see the issue right away.

> The other problem is the page's active or unevictable flag might be
> still set when freeing the page via free_page_and_swap_cache().

So what?

> The putback_lru_page() would not clear those two flags if the pages are
> released via pagevec, it sounds nothing prevents from isolating active
> or unevictable pages.

Again, why should it? vmscan is equipped to deal with this.

> However I didn't really run into these problems, just in theory by visual
> inspection.
> 
> And, it also seems unnecessary to have the pages add back to LRU again since
> they are about to be freed when reaching this point.  So, clearing active
> and unevictable flags, unlocking and dropping refcount from isolate
> instead of calling putback_lru_page() as what page cache collapse does.

Hm? But we do call putback_lru_page() on the way out. I do not follow.

> 
> Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> Cc: Hugh Dickins <hughd@google.com>
> Cc: Andrea Arcangeli <aarcange@redhat.com>
> Signed-off-by: Yang Shi <yang.shi@linux.alibaba.com>
> ---
>  mm/khugepaged.c | 10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index b679908..f42fa4e 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -673,7 +673,6 @@ static void __collapse_huge_page_copy(pte_t *pte, struct page *page,
>  			src_page = pte_page(pteval);
>  			copy_user_highpage(page, src_page, address, vma);
>  			VM_BUG_ON_PAGE(page_mapcount(src_page) != 1, src_page);
> -			release_pte_page(src_page);
>  			/*
>  			 * ptl mostly unnecessary, but preempt has to
>  			 * be disabled to update the per-cpu stats
> @@ -687,6 +686,15 @@ static void __collapse_huge_page_copy(pte_t *pte, struct page *page,
>  			pte_clear(vma->vm_mm, address, _pte);
>  			page_remove_rmap(src_page, false);
>  			spin_unlock(ptl);
> +
> +			dec_node_page_state(src_page,
> +				NR_ISOLATED_ANON + page_is_file_cache(src_page));
> +			ClearPageActive(src_page);
> +			ClearPageUnevictable(src_page);
> +			unlock_page(src_page);
> +			/* Drop refcount from isolate */
> +			put_page(src_page);
> +
>  			free_page_and_swap_cache(src_page);
>  		}
>  	}
> -- 
> 1.8.3.1
> 
> 

-- 
 Kirill A. Shutemov