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Subject[PATCH v9 06/24] mm: make pte_unmap_same compatible with SPF
Date
pte_unmap_same() is making the assumption that the page table are still
around because the mmap_sem is held.
This is no more the case when running a speculative page fault and
additional check must be made to ensure that the final page table are still
there.

This is now done by calling pte_spinlock() to check for the VMA's
consistency while locking for the page tables.

This is requiring passing a vm_fault structure to pte_unmap_same() which is
containing all the needed parameters.

As pte_spinlock() may fail in the case of a speculative page fault, if the
VMA has been touched in our back, pte_unmap_same() should now return 3
cases :
1. pte are the same (0)
2. pte are different (VM_FAULT_PTNOTSAME)
3. a VMA's changes has been detected (VM_FAULT_RETRY)

The case 2 is handled by the introduction of a new VM_FAULT flag named
VM_FAULT_PTNOTSAME which is then trapped in cow_user_page().
If VM_FAULT_RETRY is returned, it is passed up to the callers to retry the
page fault while holding the mmap_sem.

Signed-off-by: Laurent Dufour <ldufour@linux.vnet.ibm.com>
---
include/linux/mm.h | 1 +
mm/memory.c | 29 +++++++++++++++++++----------
2 files changed, 20 insertions(+), 10 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 2f3e98edc94a..b6432a261e63 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1199,6 +1199,7 @@ static inline void clear_page_pfmemalloc(struct page *page)
#define VM_FAULT_NEEDDSYNC 0x2000 /* ->fault did not modify page tables
* and needs fsync() to complete (for
* synchronous page faults in DAX) */
+#define VM_FAULT_PTNOTSAME 0x4000 /* Page table entries have changed */

#define VM_FAULT_ERROR (VM_FAULT_OOM | VM_FAULT_SIGBUS | VM_FAULT_SIGSEGV | \
VM_FAULT_HWPOISON | VM_FAULT_HWPOISON_LARGE | \
diff --git a/mm/memory.c b/mm/memory.c
index 21b1212a0892..4bc7b0bdcb40 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -2309,21 +2309,29 @@ static bool pte_map_lock(struct vm_fault *vmf)
* parts, do_swap_page must check under lock before unmapping the pte and
* proceeding (but do_wp_page is only called after already making such a check;
* and do_anonymous_page can safely check later on).
+ *
+ * pte_unmap_same() returns:
+ * 0 if the PTE are the same
+ * VM_FAULT_PTNOTSAME if the PTE are different
+ * VM_FAULT_RETRY if the VMA has changed in our back during
+ * a speculative page fault handling.
*/
-static inline int pte_unmap_same(struct mm_struct *mm, pmd_t *pmd,
- pte_t *page_table, pte_t orig_pte)
+static inline int pte_unmap_same(struct vm_fault *vmf)
{
- int same = 1;
+ int ret = 0;
+
#if defined(CONFIG_SMP) || defined(CONFIG_PREEMPT)
if (sizeof(pte_t) > sizeof(unsigned long)) {
- spinlock_t *ptl = pte_lockptr(mm, pmd);
- spin_lock(ptl);
- same = pte_same(*page_table, orig_pte);
- spin_unlock(ptl);
+ if (pte_spinlock(vmf)) {
+ if (!pte_same(*vmf->pte, vmf->orig_pte))
+ ret = VM_FAULT_PTNOTSAME;
+ spin_unlock(vmf->ptl);
+ } else
+ ret = VM_FAULT_RETRY;
}
#endif
- pte_unmap(page_table);
- return same;
+ pte_unmap(vmf->pte);
+ return ret;
}

static inline void cow_user_page(struct page *dst, struct page *src, unsigned long va, struct vm_area_struct *vma)
@@ -2913,7 +2921,8 @@ int do_swap_page(struct vm_fault *vmf)
int exclusive = 0;
int ret = 0;

- if (!pte_unmap_same(vma->vm_mm, vmf->pmd, vmf->pte, vmf->orig_pte))
+ ret = pte_unmap_same(vmf);
+ if (ret)
goto out;

entry = pte_to_swp_entry(vmf->orig_pte);
--
2.7.4
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