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

From	Vladimir Davydov <vdavydov@virtuozzo.com>
Newsgroups	linux.kernel
Subject	Re: [PATCH RESEND 7/8] pipe: account to kmemcg
Date	2016-05-25 12:40 +0200
Message-ID	<rCEoi-8jG-21@gated-at.bofh.it> (permalink)
References	<rCgvE-1kA-23@gated-at.bofh.it> <rCgvE-1kA-31@gated-at.bofh.it> <rCk6j-3mY-15@gated-at.bofh.it> <rCnGN-5K5-7@gated-at.bofh.it> <rCqOl-7UA-1@gated-at.bofh.it>
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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.

Thanks,
Vladimir

Thread

[PATCH RESEND 7/8] pipe: account to kmemcg Vladimir Davydov <vdavydov@virtuozzo.com> - 2016-05-24 11:10 +0200
  Re: [PATCH RESEND 7/8] pipe: account to kmemcg Eric Dumazet <eric.dumazet@gmail.com> - 2016-05-24 15:00 +0200
    Re: [PATCH RESEND 7/8] pipe: account to kmemcg Vladimir Davydov <vdavydov@virtuozzo.com> - 2016-05-24 18:50 +0200
      Re: [PATCH RESEND 7/8] pipe: account to kmemcg Eric Dumazet <eric.dumazet@gmail.com> - 2016-05-24 22:10 +0200
        Re: [PATCH RESEND 7/8] pipe: account to kmemcg Vladimir Davydov <vdavydov@virtuozzo.com> - 2016-05-25 12:40 +0200
          Re: [PATCH RESEND 7/8] pipe: account to kmemcg Minchan Kim <minchan@kernel.org> - 2016-05-26 09:10 +0200
            Re: [PATCH RESEND 7/8] pipe: account to kmemcg Vladimir Davydov <vdavydov@virtuozzo.com> - 2016-05-26 16:00 +0200
              Re: [PATCH RESEND 7/8] pipe: account to kmemcg Eric Dumazet <eric.dumazet@gmail.com> - 2016-05-26 16:20 +0200

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