Re: [PATCH] x86,mm: fix pte_free()

[Peter Zijlstra <peterz@infradead.org>]

On Fri, 2009-01-23 at 17:37 +0100, Peter Zijlstra wrote:
> On -rt we were seeing spurious bad page states like:
> 
> Bad page state in process 'firefox'
> page:c1bc2380 flags:0x40000000 mapping:c1bc2390 mapcount:0 count:0
> Trying to fix it up, but a reboot is needed
> Backtrace:
> Pid: 503, comm: firefox Not tainted 2.6.26.8-rt13 #3
> [<c043d0f3>] ? printk+0x14/0x19
> [<c0272d4e>] bad_page+0x4e/0x79
> [<c0273831>] free_hot_cold_page+0x5b/0x1d3
> [<c02739f6>] free_hot_page+0xf/0x11
> [<c0273a18>] __free_pages+0x20/0x2b
> [<c027d170>] __pte_alloc+0x87/0x91
> [<c027d25e>] handle_mm_fault+0xe4/0x733
> [<c043f680>] ? rt_mutex_down_read_trylock+0x57/0x63
> [<c043f680>] ? rt_mutex_down_read_trylock+0x57/0x63
> [<c0218875>] do_page_fault+0x36f/0x88a
> 
> This is the case where a concurrent fault already installed the PTE
> and
> we get to free the newly allocated one.
> 
> This is due to pgtable_page_ctor() doing the
> spin_lock_init(&page->ptl)
> which is overlaid with the {private, mapping} struct.
> 
> union {
>     struct {
>         unsigned long private;
>         struct address_space *mapping;
>     };
> #if NR_CPUS >= CONFIG_SPLIT_PTLOCK_CPUS
>     spinlock_t ptl;
> #endif
>     struct kmem_cache *slab;
>     struct page *first_page;
> };
> 
> Normally the spinlock is small enough to not stomp on page->mapping,
> but
> PREEMPT_RT=y has huge 'spin'locks.
> 
> But lockdep kernels should also be able to trigger this splat, as the
> lock tracking code grows the spinlock to cover page->mapping.
> 
> The obvious fix is calling pgtable_page_dtor() like the regular pte
> free
> path __pte_free_tlb() does.
> 
> It seems all architectures except x86 and nm10300 already do this, and
> nm10300 doesn't seem to use pgtable_page_ctor(), which suggests it
> doesn't do SMP or simply doesnt do MMU at all or something.
> 
> Signed-off-by: Peter Zijlstra <a.p.zijlsta@chello.nl>
> CC: stable@kernel.org

Now one that's not obviously borken,..

---
 arch/x86/include/asm/pgalloc.h |    1 +
 1 files changed, 1 insertions(+), 0 deletions(-)

diff --git a/arch/x86/include/asm/pgalloc.h b/arch/x86/include/asm/pgalloc.h
index cb7c151..dd14c54 100644
--- a/arch/x86/include/asm/pgalloc.h
+++ b/arch/x86/include/asm/pgalloc.h
@@ -42,6 +42,7 @@ static inline void pte_free_kernel(struct mm_struct *mm, pte_t *pte)
 
 static inline void pte_free(struct mm_struct *mm, struct page *pte)
 {
+	pgtable_page_dtor(pte);
 	__free_page(pte);
 }