userfaultfd: non-cooperative: add event for memory unmaps

When a non-cooperative userfaultfd monitor copies pages in the
background, it may encounter regions that were already unmapped.
Addition of UFFD_EVENT_UNMAP allows the uffd monitor to track precisely
changes in the virtual memory layout.

Since there might be different uffd contexts for the affected VMAs, we
first should create a temporary representation for the unmap event for
each uffd context and then notify them one by one to the appropriate
userfault file descriptors.

The event notification occurs after the mmap_sem has been released.

[arnd@arndb.de: fix nommu build]
  Link: http://lkml.kernel.org/r/20170203165141.3665284-1-arnd@arndb.de
[mhocko@suse.com: fix nommu build]
  Link: http://lkml.kernel.org/r/20170202091503.GA22823@dhcp22.suse.cz
Link: http://lkml.kernel.org/r/1485542673-24387-3-git-send-email-rppt@linux.vnet.ibm.com
Signed-off-by: Mike Rapoport <rppt@linux.vnet.ibm.com>
Signed-off-by: Michal Hocko <mhocko@suse.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Hillf Danton <hillf.zj@alibaba-inc.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Pavel Emelyanov <xemul@virtuozzo.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 65 insertions, 0 deletions

diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
index 8fe601b4875e..4c78458ea78d 100644
--- a/fs/userfaultfd.c
+++ b/fs/userfaultfd.c
@@ -71,6 +71,13 @@ struct userfaultfd_fork_ctx {
         struct list_head list;
 };
 
+struct userfaultfd_unmap_ctx {
+        struct userfaultfd_ctx *ctx;
+        unsigned long start;
+        unsigned long end;
+        struct list_head list;
+};
+
 struct userfaultfd_wait_queue {
         struct uffd_msg msg;
         wait_queue_t wq;
@@ -709,6 +716,64 @@ void userfaultfd_remove(struct vm_area_struct *vma,
         down_read(&mm->mmap_sem);
 }
 
+static bool has_unmap_ctx(struct userfaultfd_ctx *ctx, struct list_head *unmaps,
+                          unsigned long start, unsigned long end)
+{
+        struct userfaultfd_unmap_ctx *unmap_ctx;
+
+        list_for_each_entry(unmap_ctx, unmaps, list)
+                if (unmap_ctx->ctx == ctx && unmap_ctx->start == start &&
+                    unmap_ctx->end == end)
+                        return true;
+
+        return false;
+}
+
+int userfaultfd_unmap_prep(struct vm_area_struct *vma,
+                           unsigned long start, unsigned long end,
+                           struct list_head *unmaps)
+{
+        for ( ; vma && vma->vm_start < end; vma = vma->vm_next) {
+                struct userfaultfd_unmap_ctx *unmap_ctx;
+                struct userfaultfd_ctx *ctx = vma->vm_userfaultfd_ctx.ctx;
+
+                if (!ctx || !(ctx->features & UFFD_FEATURE_EVENT_UNMAP) ||
+                    has_unmap_ctx(ctx, unmaps, start, end))
+                        continue;
+
+                unmap_ctx = kzalloc(sizeof(*unmap_ctx), GFP_KERNEL);
+                if (!unmap_ctx)
+                        return -ENOMEM;
+
+                userfaultfd_ctx_get(ctx);
+                unmap_ctx->ctx = ctx;
+                unmap_ctx->start = start;
+                unmap_ctx->end = end;
+                list_add_tail(&unmap_ctx->list, unmaps);
+        }
+
+        return 0;
+}
+
+void userfaultfd_unmap_complete(struct mm_struct *mm, struct list_head *uf)
+{
+        struct userfaultfd_unmap_ctx *ctx, *n;
+        struct userfaultfd_wait_queue ewq;
+
+        list_for_each_entry_safe(ctx, n, uf, list) {
+                msg_init(&ewq.msg);
+
+                ewq.msg.event = UFFD_EVENT_UNMAP;
+                ewq.msg.arg.remove.start = ctx->start;
+                ewq.msg.arg.remove.end = ctx->end;
+
+                userfaultfd_event_wait_completion(ctx->ctx, &ewq);
+
+                list_del(&ctx->list);
+                kfree(ctx);
+        }
+}
+
 static int userfaultfd_release(struct inode *inode, struct file *file)
 {
         struct userfaultfd_ctx *ctx = file->private_data;