Re: [PATCH RESEND 7/8] pipe: account to kmemcg

[Minchan Kim <minchan@kernel.org>]

On Wed, May 25, 2016 at 01:30:11PM +0300, Vladimir Davydov wrote:
> On Tue, May 24, 2016 at 01:04:33PM -0700, Eric Dumazet wrote:
> > 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:
> > > ...
> > > > > +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 ?
> 
> Each pipe_buffer holds a reference to its page. If we find page's
> refcount to be 1 here, then it can be referenced only by our
> pipe_buffer. And the refcount cannot be increased by a parallel thread,
> because we hold pipe_lock, which rules out splice, and otherwise it's
> impossible to reach the page as it is not on lru. That said, I think I
> guarantee that this should be safe.

I don't know kmemcg internal and pipe stuff so my comment might be
totally crap.

No one cannot guarantee any CPU cannot held a reference of a page.
Look at get_page_unless_zero usecases.

1. balloon_page_isolate

It can hold a reference in random page and then verify the page
is balloon page. Otherwise, just put.

2. page_idle_get_page

It has PageLRU check but it's racy so it can hold a reference
of randome page and then verify within zone->lru_lock. If it's
not LRU page, just put.