linux-kernel - Re: [PATCH V5 2/6] mm: don't assume anonymous pages have SwapBacked flag

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Date:   Mon, 27 Feb 2017 15:35:34 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Shaohua Li <shli@...com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Kernel-team@...com, minchan@...nel.org, hughd@...gle.com,
        hannes@...xchg.org, riel@...hat.com, mgorman@...hsingularity.net,
        akpm@...ux-foundation.org
Subject: Re: [PATCH V5 2/6] mm: don't assume anonymous pages have SwapBacked
 flag

On Fri 24-02-17 13:31:45, Shaohua Li wrote:
> There are a few places the code assumes anonymous pages should have
> SwapBacked flag set. MADV_FREE pages are anonymous pages but we are
> going to add them to LRU_INACTIVE_FILE list and clear SwapBacked flag
> for them. The assumption doesn't hold any more, so fix them.
> 
> Cc: Michal Hocko <mhocko@...e.com>
> Cc: Minchan Kim <minchan@...nel.org>
> Cc: Hugh Dickins <hughd@...gle.com>
> Cc: Rik van Riel <riel@...hat.com>
> Cc: Mel Gorman <mgorman@...hsingularity.net>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Acked-by: Johannes Weiner <hannes@...xchg.org>
> Signed-off-by: Shaohua Li <shli@...com>

Looks good to me.
[...]
> index 96eb85c..c621088 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -1416,7 +1416,8 @@ static int try_to_unmap_one(struct page *page, struct vm_area_struct *vma,
>  			 * Store the swap location in the pte.
>  			 * See handle_pte_fault() ...
>  			 */
> -			VM_BUG_ON_PAGE(!PageSwapCache(page), page);
> +			VM_BUG_ON_PAGE(!PageSwapCache(page) && PageSwapBacked(page),
> +				page);

just this part makes me scratch my head. I really do not understand what
kind of problem it tries to prevent from, maybe I am missing something
obvoious...

>  
>  			if (!PageDirty(page)) {
>  				/* It's a freeable page by MADV_FREE */
> -- 
> 2.9.3
> 

-- 
Michal Hocko
SUSE Labs