Re: [PATCH 1/1] Fixup write permission of TLB on powerpc e500 core

From: Shan Hai
Date: Sat Jul 16 2011 - 10:51:00 EST

On 07/15/2011 06:23 AM, Peter Zijlstra wrote:
On Fri, 2011-07-15 at 16:07 +0800, Shan Hai wrote:
The kernel has no write permission on COW pages by default on e500 core, this
will cause endless loop in futex_lock_pi, because futex code assumes the kernel
has write permission on COW pages. Grant write permission to the kernel on COW
pages when access violation page fault occurs.

Signed-off-by: Shan Hai<haishan.bai@xxxxxxxxx>
arch/powerpc/include/asm/futex.h | 11 ++++++++++-
arch/powerpc/include/asm/tlb.h | 25 +++++++++++++++++++++++++
2 files changed, 35 insertions(+), 1 deletions(-)

diff --git a/arch/powerpc/include/asm/futex.h b/arch/powerpc/include/asm/futex.h
index c94e4a3..54c3e74 100644
--- a/arch/powerpc/include/asm/futex.h
+++ b/arch/powerpc/include/asm/futex.h
@@ -8,6 +8,7 @@

#define __futex_atomic_op(insn, ret, oldval, uaddr, oparg) \
__asm__ __volatile ( \
@@ -113,7 +114,15 @@ futex_atomic_cmpxchg_inatomic(u32 *uval, u32 __user *uaddr,
: "cc", "memory");

*uval = prev;
- return ret;
+ /* Futex assumes the kernel has permission to write to
+ * COW pages, grant the kernel write permission on COW
+ * pages because it has none by default.
+ */
+ if (ret == -EFAULT)
+ __tlb_fixup_write_permission(current->mm, (unsigned long)uaddr);
+ return ret;

#endif /* __KERNEL__ */
diff --git a/arch/powerpc/include/asm/tlb.h b/arch/powerpc/include/asm/tlb.h
index e2b428b..3863c6a 100644
--- a/arch/powerpc/include/asm/tlb.h
+++ b/arch/powerpc/include/asm/tlb.h
@@ -45,5 +45,30 @@ static inline void __tlb_remove_tlb_entry(struct mmu_gather *tlb, pte_t *ptep,

+/* Grant write permission to the kernel on a page. */
+static inline void __tlb_fixup_write_permission(struct mm_struct *mm,
+ unsigned long address)
+#if defined(CONFIG_FSL_BOOKE)
+ /* Grant write permission to the kernel on a page by setting TLB.SW
+ * bit, the bit setting operation is tricky here, calling
+ * handle_mm_fault with FAULT_FLAG_WRITE causes _PAGE_DIRTY bit of
+ * the pte to be set, the _PAGE_DIRTY of the pte is translated into
+ * TLB.SW on Powerpc e500 core.
+ */
+ struct vm_area_struct *vma;
+ vma = find_vma(mm, address);
Uhm, find_vma() needs mmap_sem, and futex_atomic_cmpxchg_inatomic() is
most certainly not called with that lock held.

+ if (likely(vma)) {
+ /* only fixup present page */
+ if (follow_page(vma, address, FOLL_WRITE)) {
+ handle_mm_fault(mm, vma, address, FAULT_FLAG_WRITE);
So how can this toggle your sw dirty/young tracking, that's pretty much
what gup(.write=1) does too!

That's right the gup(.write=1) want to do the same thing as the
above code snippet, but it failed for the following reason:
because the get_user_pages() would not dirty pte for the reason
the follow_page() returns not NULL on *present* and *writable*
page, the page which holds the lock is present because its a shared page,
writable because demand paging set that up so for shared
writable page, so the handle_mm_fault() in the __get_user_page()
could not be called.

Why the above code could do the same task, because by calling
handle_mm_fault() will set pte dirty by
[do_annonymous_page(), memory.c]
if (vma->vm_flags & VM_WRITE)
entry = pte_mkwrite(pte_mkdirty(entry));

Shan Hai

+ flush_tlb_page(vma, address);
+ }
+ }
#endif /* __KERNEL__ */
#endif /* __ASM_POWERPC_TLB_H */

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