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Date:   Sat, 9 Jan 2021 19:32:24 +0000
From:   Matthew Wilcox <willy@infradead.org>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Jason Gunthorpe <jgg@ziepe.ca>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Linux-MM <linux-mm@kvack.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Yu Zhao <yuzhao@google.com>, Andy Lutomirski <luto@kernel.org>,
        Peter Xu <peterx@redhat.com>,
        Pavel Emelyanov <xemul@openvz.org>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Mike Rapoport <rppt@linux.vnet.ibm.com>,
        Minchan Kim <minchan@kernel.org>,
        Will Deacon <will@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Hugh Dickins <hughd@google.com>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Oleg Nesterov <oleg@redhat.com>, Jann Horn <jannh@google.com>,
        Kees Cook <keescook@chromium.org>,
        John Hubbard <jhubbard@nvidia.com>,
        Leon Romanovsky <leonro@nvidia.com>, Jan Kara <jack@suse.cz>,
        Kirill Tkhai <ktkhai@virtuozzo.com>
Subject: Re: [PATCH 0/2] page_count can't be used to decide when wp_page_copy
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On Thu, Jan 07, 2021 at 01:05:19PM -0800, Linus Torvalds wrote:
> Side note, and not really related to UFFD, but the mmap_sem in
> general: I was at one point actually hoping that we could make the
> mmap_sem a spinlock, or at least make the rule be that we never do any
> IO under it. At which point a write lock hopefully really shouldn't be
> such a huge deal.

There's a (small) group of us working towards that.  It has some
prerequisites, but where we're hoping to go currently:

 - Replace the vma rbtree with a b-tree protected with a spinlock
 - Page faults walk the b-tree under RCU, like peterz/laurent's SPF patchset
 - If we need to do I/O, take a refcount on the VMA

After that, we can gradually move things out from mmap_sem protection
to just the vma tree spinlock, or whatever makes sense for them.  In a
very real way the mmap_sem is the MM layer's BKL.