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From: Bui Quang Minh <minhquangbui99@gmail.com>
To: Mike Rapoport <rppt@kernel.org>
Cc: Michal Hocko <mhocko@suse.com>,
	akpm@linux-foundation.org, linux-mm@kvack.org,
	linux-kernel@vger.kernel.org,
	Andrea Arcangeli <aarcange@redhat.com>,
	Peter Xu <peterx@redhat.com>
Subject: Re: [PATCH] userfaultfd: Write protect when virtual memory range has no page table entry
Date: Tue, 23 Mar 2021 09:48:03 +0700	[thread overview]
Message-ID: <20210323024803.GA3365@local> (raw)
In-Reply-To: <YFiU9YWbYpLnlnde@kernel.org>

On Mon, Mar 22, 2021 at 03:00:37PM +0200, 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.

Yes, you are right. I saw a patch from Peter to linux-man and saw that

	"When there is only UFFDIO_REGISTER_MODE_WP registered, the userspace
	will not receive any message when a missing page is written"

It's my mistake that I didn't look at the documentation carefully when playing
around.

Thanks,
Quang Minh.

  parent reply	other threads:[~2021-03-23  2:48 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
2021-03-31 14:49       ` Michal Hocko
2021-04-01  0:24         ` Andrew Morton
2021-03-23  2:48     ` Bui Quang Minh [this message]
2021-03-23 15:12       ` Peter Xu

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