Re: [PATCH] mm,thp: recheck each page before collapsing file THP

[Kirill A. Shutemov]

On Thu, Oct 17, 2019 at 10:08:32PM -0700, Song Liu wrote:
> In collapse_file(), after locking the page, it is necessary to recheck
> that the page is up-to-date, clean, and pointing to the proper mapping.
> If any check fails, abort the collapse.
> 
> Fixes: 99cb0dbd47a1 ("mm,thp: add read-only THP support for (non-shmem) FS")
> Cc: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
> Cc: Hugh Dickins <hughd@xxxxxxxxxx>
> Cc: William Kucharski <william.kucharski@xxxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Signed-off-by: Song Liu <songliubraving@xxxxxx>
> ---
>  mm/khugepaged.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 0a1b4b484ac5..7da49b643c4d 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -1619,6 +1619,14 @@ static void collapse_file(struct mm_struct *mm,
>  				result = SCAN_PAGE_LOCK;
>  				goto xa_locked;
>  			}
> +
> +			/* double check the page is correct and clean */
> +			if (unlikely(!PageUptodate(page)) ||
> +			    unlikely(PageDirty(page)) ||
> +			    unlikely(page->mapping != mapping)) {
> +				result = SCAN_FAIL;
> +				goto out_unlock;
> +			}
>  		}
>  
>  		/*

Hm. But why only for !is_shmem? Or I read it wrong?

-- 
 Kirill A. Shutemov