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Date:   Tue, 22 Sep 2020 14:13:06 -0400
From:   Peter Xu <peterx@redhat.com>
To:     Oleg Nesterov <oleg@redhat.com>
Cc:     linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        Jason Gunthorpe <jgg@ziepe.ca>,
        Andrew Morton <akpm@linux-foundation.org>,
        Jan Kara <jack@suse.cz>, Michal Hocko <mhocko@suse.com>,
        Kirill Tkhai <ktkhai@virtuozzo.com>,
        Kirill Shutemov <kirill@shutemov.name>,
        Hugh Dickins <hughd@google.com>,
        Christoph Hellwig <hch@lst.de>,
        Andrea Arcangeli <aarcange@redhat.com>,
        John Hubbard <jhubbard@nvidia.com>,
        Leon Romanovsky <leonro@nvidia.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Jann Horn <jannh@google.com>
Subject: Re: [PATCH 3/5] mm: Rework return value for copy_one_pte()
Message-ID: <20200922181306.GJ19098@xz-x1>
References: <20200921211744.24758-1-peterx@redhat.com>
 <20200921211744.24758-4-peterx@redhat.com>
 <20200922100840.GA11679@redhat.com>
 <20200922101815.GB11679@redhat.com>
 <20200922153612.GF19098@xz-x1>
 <20200922154845.GE11679@redhat.com>
 <20200922160330.GH19098@xz-x1>
 <20200922165354.GG11679@redhat.com>
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In-Reply-To: <20200922165354.GG11679@redhat.com>
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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