linux-kernel - Re: [PATCH 2/2] mm: soft_dirty: userfaultfd: introduce wrprotect_tlb_flush

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Message-ID: <CAHk-=whg-91=EF=8=ayyDQGx_3iuWKp3aHUkDCDkgUb15Yh8AQ@mail.gmail.com>
Date:   Thu, 7 Jan 2021 12:17:40 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Andrea Arcangeli <aarcange@...hat.com>
Cc:     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>,
        Jason Gunthorpe <jgg@...pe.ca>, Jan Kara <jack@...e.cz>,
        Kirill Tkhai <ktkhai@...tuozzo.com>
Subject: Re: [PATCH 2/2] mm: soft_dirty: userfaultfd: introduce wrprotect_tlb_flush_pending

On Thu, Jan 7, 2021 at 12:04 PM Andrea Arcangeli <aarcange@...hat.com> wrote:
>
> However there are two cases that could wrprotecting exclusive anon
> pages with only the mmap_read_lock:

I still think the real fix is "Don't do that then", and just take the
write lock.

The UFFDIO_WRITEPROTECT case simply isn't that critical. It's not a
normal operation. Same goes for softdirty.

Why have those become _so_ magical that they can break the VM for
everybody else?

         Linus