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Date:   Wed, 23 Sep 2020 08:52:38 +0200
From:   Oleg Nesterov <oleg@redhat.com>
To:     Peter Xu <peterx@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: <20200923065237.GA21395@redhat.com>
References: <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>
 <20200922181306.GJ19098@xz-x1>
 <20200922182317.GH11679@redhat.com>
 <20200922184905.GM19098@xz-x1>
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On 09/22, Peter Xu wrote:
>
> On Tue, Sep 22, 2020 at 08:23:18PM +0200, Oleg Nesterov wrote:
> >
> > But I still think that !pte_none() -> pte_none() transition is not possible
> > under mmap_write_lock()...
> >
> > OK, let me repeat I don't understans these code paths enough, let me reword:
> > I don't see how this transition is possible.
>
> Though I guess I'll keep my wording, because I still think it's accurate to
> me. :)
>
> Can we e.g. punch a page hole without changing vmas?

punch a hole? I don't know what does it mean...

However, I think you are right anyway. I forgot that (at least) truncate can
clear this pte without mmap_sem after pte_unmap_unlock().

So I think you are right, the current code is wrong too.

Thanks!

Oleg.