linux-kernel - Re: [PATCH 3/5] mm: Rework return value for copy_one

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Message-ID: <20200922181306.GJ19098@xz-x1>
Date:   Tue, 22 Sep 2020 14:13:06 -0400
From:   Peter Xu <peterx@...hat.com>
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Jason Gunthorpe <jgg@...pe.ca>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jan Kara <jack@...e.cz>, Michal Hocko <mhocko@...e.com>,
        Kirill Tkhai <ktkhai@...tuozzo.com>,
        Kirill Shutemov <kirill@...temov.name>,
        Hugh Dickins <hughd@...gle.com>,
        Christoph Hellwig <hch@....de>,
        Andrea Arcangeli <aarcange@...hat.com>,
        John Hubbard <jhubbard@...dia.com>,
        Leon Romanovsky <leonro@...dia.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Jann Horn <jannh@...gle.com>
Subject: Re: [PATCH 3/5] mm: Rework return value for copy_one_pte()

On Tue, Sep 22, 2020 at 06:53:55PM +0200, Oleg Nesterov wrote:
> On 09/22, Peter Xu wrote:
> >
> > On Tue, Sep 22, 2020 at 05:48:46PM +0200, Oleg Nesterov wrote:
> > > > However since I didn't change this logic in this patch, it probably means this
> > > > bug is also in the original code before this series...  I'm thinking maybe I
> > > > should prepare a standalone patch to clear the swp_entry_t and cc stable.
> > >
> > > Well, if copy_one_pte(src_pte) hits a swap entry and returns entry.val != 0, then
> > > pte_none(*src_pte) is not possible after restart? This means that copy_one_pte()
> > > will be called at least once.
> >
> > Note that we've released the page table locks, so afaict the old swp entry can
> > be gone under us when we go back to the "do" loop... :)
> 
> But how?
> 
> I am just curious, I don't understand this code enough.

Me neither.

The point is I think we can't assume *src_pte will read the same if we have
released the src_ptl in copy_pte_range(), because imho the src_ptl is the only
thing to protect it.  Or to be more explicit, we need pte_alloc_map_lock() to
read a stable pmd/pte or before update (since src_ptl itself could change too).

Thanks,

-- 
Peter Xu