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.
next prev 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
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20210323024803.GA3365@local \
--to=minhquangbui99@gmail.com \
--cc=aarcange@redhat.com \
--cc=akpm@linux-foundation.org \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-mm@kvack.org \
--cc=mhocko@suse.com \
--cc=peterx@redhat.com \
--cc=rppt@kernel.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).