linux-kernel - Re: [PATCH 1/5] mm/vmscan: put the redirtied MADV

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Date:   Tue, 13 Jul 2021 14:34:11 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Miaohe Lin <linmiaohe@...wei.com>
Cc:     Michal Hocko <mhocko@...e.com>, akpm@...ux-foundation.org,
        hannes@...xchg.org, vbabka@...e.cz, axboe@...nel.dk,
        iamjoonsoo.kim@....com, alexs@...nel.org, apopple@...dia.com,
        minchan@...nel.org, david@...hat.com, shli@...com,
        hillf.zj@...baba-inc.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/5] mm/vmscan: put the redirtied MADV_FREE pages back to
 anonymous LRU list

On Tue, Jul 13, 2021 at 09:13:51PM +0800, Miaohe Lin wrote:
> >> When the MADV_FREE pages are redirtied before they could be reclaimed, the pages
> >> should be put back to anonymous LRU list by setting SwapBacked flag, thus the
> >> pages will be reclaimed in normal swapout way.
> > 
> > Agreed. But the question is why this needs an explicit handling here
> > when we already do handle this case when trying to unmap the page.
> 
> This makes me think more. It seems even the page_ref_freeze call is guaranteed to
> success as no one can grab the page refcnt after the page is successfully unmapped.

NO!  This is wrong.  Every page can have its refcount speculatively raised
(and then lowered).  The two prime candidates for this are lockless GUP
and page cache lookups, but there can be others too.