linux-kernel - Re: [PATCH 0/2] page_count can't be used to decide when wp_page

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Date:   Thu, 7 Jan 2021 17:02:50 -0500
From:   Andrea Arcangeli <aarcange@...hat.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Jason Gunthorpe <jgg@...pe.ca>, Linux-MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Yu Zhao <yuzhao@...gle.com>, Andy Lutomirski <luto@...nel.org>,
        Peter Xu <peterx@...hat.com>,
        Pavel Emelyanov <xemul@...nvz.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        Minchan Kim <minchan@...nel.org>,
        Will Deacon <will@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Hugh Dickins <hughd@...gle.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Matthew Wilcox <willy@...radead.org>,
        Oleg Nesterov <oleg@...hat.com>, Jann Horn <jannh@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        John Hubbard <jhubbard@...dia.com>,
        Leon Romanovsky <leonro@...dia.com>, Jan Kara <jack@...e.cz>,
        Kirill Tkhai <ktkhai@...tuozzo.com>
Subject: Re: [PATCH 0/2] page_count can't be used to decide when wp_page_copy

On Thu, Jan 07, 2021 at 01:05:19PM -0800, Linus Torvalds wrote:
> I think those would very much be worth fixing, so that if
> UFFDIO_WRITEPROTECT taking the mmapo_sem for writing causes problems,
> we can _fix_ those problems.
> 
> But I think it's entirely wrong to treat UFFDIO_WRITEPROTECT as
> specially as Andrea seems to want to treat it. Particularly with
> absolutely zero use cases to back it up.

Again for the record: there's nothing at all special in
UFFDIO_WRITEPROTECT in this respect.

If you could stop mentioning UFFDIO_WRITEPROTECT and only focus on
softdirty/clear_refs, maybe you wouldn't think my judgment is biased
towards clear_refs/softdirty too.

You can imagine the side effects of page_count doing a COW
erroneously, as corollary of the fact that KSM won't ever allow to
merge two pages if one of them is under GUP pin. Why is that?

Thanks,
Andrea