Re: [PATCH v2 1/1] mm: fix the theoretical compound_lock() vsprep_new_page() race

[Andrew Morton]

On Fri, 3 Jan 2014 20:55:47 +0100 Oleg Nesterov <oleg@xxxxxxxxxx> 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?

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