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Date: Fri, 3 Jan 2014 13:00:23 -0800
From: Andrew Morton <akpm@linux-foundation.org>
To: Oleg Nesterov <oleg@redhat.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>,
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        Martin Schwidefsky <schwidefsky@de.ibm.com>,
        Heiko Carstens <heiko.carstens@de.ibm.com>
Subject: Re: [PATCH v2 1/1] mm: fix the theoretical compound_lock() vs
 prep_new_page() race
Message-Id: <20140103130023.fdbf96fc95c702bf63871b56@linux-foundation.org>
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On Fri, 3 Jan 2014 20:55:47 +0100 Oleg Nesterov <oleg@redhat.com> wrote:

> get/put_page(thp_tail) paths do get_page_unless_zero(page_head) +
> compound_lock(). In theory this page_head can be already freed and
> reallocated as alloc_pages(__GFP_COMP, smaller_order). In this case
> get_page_unless_zero() can succeed right after set_page_refcounted(),
> and compound_lock() can race with the non-atomic __SetPageHead().

Would be useful to mention that these things are happening inside
prep_compound_opage() (yes?).

> Perhaps we should rework the thp locking (under discussion), but
> until then this patch moves set_page_refcounted() and adds wmb()
> to ensure that page->_count != 0 comes as a last change.
> 
> I am not sure about other callers of set_page_refcounted(), but at
> first glance they look fine to me.

I don't get it.  We're in prep_new_page() - this page is freshly
allocated and no other thread yet has any means by which to look it up
and start fiddling with it?