On Tue, 2016-05-24 at 19:13 +0300, Vladimir Davydov wrote:

On Tue, May 24, 2016 at 05:59:02AM -0700, Eric Dumazet wrote:
...

quoted

quoted

+static int anon_pipe_buf_steal(struct pipe_inode_info *pipe,
+			       struct pipe_buffer *buf)
+{
+	struct page *page = buf->page;
+
+	if (page_count(page) == 1) {

This looks racy : some cpu could have temporarily elevated page count.

All pipe operations (pipe_buf_operations->get, ->release, ->steal) are
supposed to be called under pipe_lock. So, if we see a pipe_buffer->page
with refcount of 1 in ->steal, that means that we are the only its user
and it can't be spliced to another pipe.

In fact, I just copied the code from generic_pipe_buf_steal, adding
kmemcg related checks along the way, so it should be fine.

So you guarantee that no other cpu might have done
get_page_unless_zero() right before this test ?



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