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    SubjectRe: [PATCH] x86,mm: fix pte_free()
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    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);
    }




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