linux-kernel - Re: Recurring warning in page_copy_sane (inside copy_page_to_iter) when running stress tests involving drop

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Message-ID: <20190515151814.GD31704@bombadil.infradead.org>
Date:   Wed, 15 May 2019 08:18:14 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     Lech Perczak <l.perczak@...lintechnologies.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Piotr Figiel <p.figiel@...lintechnologies.com>,
        Krzysztof Drobiński 
        <k.drobinski@...lintechnologies.com>,
        Pawel Lenkow <p.lenkow@...lintechnologies.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: Recurring warning in page_copy_sane (inside copy_page_to_iter)
 when running stress tests involving drop_caches

On Wed, May 15, 2019 at 08:02:17AM -0700, Eric Dumazet wrote:
> On Wed, May 15, 2019 at 7:43 AM Matthew Wilcox <willy@...radead.org> wrote:
> > You're seeing a race between page_address(page) being called twice.
> > Between those two calls, something has caused the page to be removed from
> > the page_address_map() list.  Eric's patch avoids calling page_address(),
> > so apply it and be happy.
> 
> Hmm... wont the kmap_atomic() done later, after page_copy_sane() would
> suffer from the race ?
> 
> It seems there is a real bug somewhere to fix.

No.  page_address() called before the kmap_atomic() will look through
the list of mappings and see if that page is mapped somewhere.  We unmap
lazily, so all it takes to trigger this race is that the page _has_
been mapped before, and its mapping gets torn down during this call.

While the page is kmapped, its mapping cannot be torn down.