Re: [PATCH v2 3/5] userfaultfd: non-cooperative: add event for exit() notification

[Mike Rapoport <rppt@linux.vnet.ibm.com>]

Hello Andrew,

On Tue, Jan 31, 2017 at 04:41:32PM -0800, Andrew Morton wrote:
> On Fri, 27 Jan 2017 20:44:31 +0200 Mike Rapoport <rppt@linux.vnet.ibm.com> wrote:
> 
> > Allow userfaultfd monitor track termination of the processes that have
> > memory backed by the uffd.
> > 
> > --- a/fs/userfaultfd.c
> > +++ b/fs/userfaultfd.c
> > @@ -774,6 +774,30 @@ void userfaultfd_unmap_complete(struct mm_struct *mm, struct list_head *uf)
> >  	}
> >  }
> >  
> > +void userfaultfd_exit(struct mm_struct *mm)
> > +{
> > +	struct vm_area_struct *vma = mm->mmap;
> > +
> > +	while (vma) {
> > +		struct userfaultfd_ctx *ctx = vma->vm_userfaultfd_ctx.ctx;
> > +
> > +		if (ctx && (ctx->features & UFFD_FEATURE_EVENT_EXIT)) {
> > +			struct userfaultfd_wait_queue ewq;
> > +
> > +			userfaultfd_ctx_get(ctx);
> > +
> > +			msg_init(&ewq.msg);
> > +			ewq.msg.event = UFFD_EVENT_EXIT;
> > +
> > +			userfaultfd_event_wait_completion(ctx, &ewq);
> > +
> > +			ctx->features &= ~UFFD_FEATURE_EVENT_EXIT;
> > +		}
> > +
> > +		vma = vma->vm_next;
> > +	}
> > +}
> 
> And we can do the vma walk without locking because the caller (exit_mm)
> knows it now has exclusive access.  Worth a comment?

I've just used your wording, seems to me neat and to the point.
 
>From 74af75b062dcb2dc3cc8d2b57f9388c22c2b89e5 Mon Sep 17 00:00:00 2001
From: Mike Rapoport <rppt@linux.vnet.ibm.com>
Date: Thu, 2 Feb 2017 15:50:14 +0200
Subject: [PATCH] userfaultfd: exit event: add comment about lockless vma walk

Signed-off-by: Mike Rapoport <rppt@linux.vnet.ibm.com>
---
 fs/userfaultfd.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
index 6587f40..3c421d0 100644
--- a/fs/userfaultfd.c
+++ b/fs/userfaultfd.c
@@ -778,6 +778,10 @@ void userfaultfd_exit(struct mm_struct *mm)
 {
 	struct vm_area_struct *vma = mm->mmap;
 
+	/*
+	 * We can do the vma walk without locking because the caller
+	 * (exit_mm) knows it now has exclusive access
+	 */
 	while (vma) {
 		struct userfaultfd_ctx *ctx = vma->vm_userfaultfd_ctx.ctx;
 
-- 
1.9.1