linux-kernel - Re: [PATCH] userfaultfd: Write protect when virtual memory range has no page table entry

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Date:   Mon, 22 Mar 2021 14:49:34 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Mike Rapoport <rppt@...nel.org>
Cc:     Bui Quang Minh <minhquangbui99@...il.com>,
        akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        Andrea Arcangeli <aarcange@...hat.com>
Subject: Re: [PATCH] userfaultfd: Write protect when virtual memory range has
 no page table entry

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