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

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Message-ID: <20170227161022.GA62304@shli-mbp.local>
Date:   Mon, 27 Feb 2017 08:10:24 -0800
From:   Shaohua Li <shli@...com>
To:     Michal Hocko <mhocko@...nel.org>
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 Mon, Feb 27, 2017 at 03:35:34PM +0100, Michal Hocko wrote:
> 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...

Just check a page which isn't lazyfree but wrongly enters here without swap
entry. Or maybe you suggest we delete this statement?

Thanks,
Shaohua