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

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Date:   Wed, 23 Sep 2020 08:52:38 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Peter Xu <peterx@...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 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.