From: Michal Hocko <mhocko@suse.com>
To: Mike Rapoport <rppt@kernel.org>
Cc: Bui Quang Minh <minhquangbui99@gmail.com>,
akpm@linux-foundation.org, linux-mm@kvack.org,
linux-kernel@vger.kernel.org,
Andrea Arcangeli <aarcange@redhat.com>
Subject: Re: [PATCH] userfaultfd: Write protect when virtual memory range has no page table entry
Date: Mon, 22 Mar 2021 14:49:34 +0100 [thread overview]
Message-ID: <YFigbjaTT+YEEAO6@dhcp22.suse.cz> (raw)
In-Reply-To: <YFiU9YWbYpLnlnde@kernel.org>
On Mon 22-03-21 15:00:37, Mike Rapoport wrote:
> On Mon, Mar 22, 2021 at 11:14:37AM +0100, Michal Hocko wrote:
> > Le'ts Andrea and Mike
> >
> > On Fri 19-03-21 22:24:28, Bui Quang Minh wrote:
> > > userfaultfd_writeprotect() use change_protection() to clear write bit in
> > > page table entries (pte/pmd). So, later write to this virtual address
> > > range causes a page fault, which is then handled by userspace program.
> > > However, change_protection() has no effect when there is no page table
> > > entries associated with that virtual memory range (a newly mapped memory
> > > range). As a result, later access to that memory range causes allocating a
> > > page table entry with write bit still set (due to VM_WRITE flag in
> > > vma->vm_flags).
> > >
> > > Add checks for VM_UFFD_WP in vma->vm_flags when allocating new page table
> > > entry in missing page table entry page fault path.
> >
> > From the above it is not really clear whether this is a usability
> > problem or a bug of the interface.
>
> I'd say it's usability/documentation clarity issue.
> Userspace can register an area with
>
> UFFDIO_REGISTER_MODE_MISSING | UFFDIO_REGISTER_MODE_WP
>
> and then it will be notified either when page table has no entry for a
> virtual address or when there is a write to a write protected address.
Thanks for the clarification! I have suspected this to be the case but
I am not really familiar with the interface to have any strong statement
here. Maybe we want to document this explicitly.
--
Michal Hocko
SUSE Labs
next prev parent reply other threads:[~2021-03-22 13:49 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-03-19 15:24 [PATCH] userfaultfd: Write protect when virtual memory range has no page table entry Bui Quang Minh
2021-03-20 16:32 ` Andrew Morton
2021-03-22 10:14 ` Michal Hocko
2021-03-22 10:23 ` Mike Rapoport
2021-03-22 13:00 ` Mike Rapoport
2021-03-22 13:27 ` Peter Xu
2021-03-22 13:49 ` Michal Hocko [this message]
2021-03-31 14:49 ` Michal Hocko
2021-04-01 0:24 ` Andrew Morton
2021-03-23 2:48 ` Bui Quang Minh
2021-03-23 15:12 ` Peter Xu
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