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Subject[PATCH v12 07/31] 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.

Acked-by: David Rientjes <rientjes@google.com>
Signed-off-by: Laurent Dufour <ldufour@linux.ibm.com>
---
include/linux/mm_types.h | 6 +++++-
mm/memory.c | 37 +++++++++++++++++++++++++++----------
2 files changed, 32 insertions(+), 11 deletions(-)

diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 8ec38b11b361..fd7d38ee2e33 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -652,6 +652,8 @@ typedef __bitwise unsigned int vm_fault_t;
* @VM_FAULT_NEEDDSYNC: ->fault did not modify page tables and needs
* fsync() to complete (for synchronous page faults
* in DAX)
+ * @VM_FAULT_PTNOTSAME Page table entries have changed during a
+ * speculative page fault handling.
* @VM_FAULT_HINDEX_MASK: mask HINDEX value
*
*/
@@ -669,6 +671,7 @@ enum vm_fault_reason {
VM_FAULT_FALLBACK = (__force vm_fault_t)0x000800,
VM_FAULT_DONE_COW = (__force vm_fault_t)0x001000,
VM_FAULT_NEEDDSYNC = (__force vm_fault_t)0x002000,
+ VM_FAULT_PTNOTSAME = (__force vm_fault_t)0x004000,
VM_FAULT_HINDEX_MASK = (__force vm_fault_t)0x0f0000,
};

@@ -693,7 +696,8 @@ enum vm_fault_reason {
{ VM_FAULT_RETRY, "RETRY" }, \
{ VM_FAULT_FALLBACK, "FALLBACK" }, \
{ VM_FAULT_DONE_COW, "DONE_COW" }, \
- { VM_FAULT_NEEDDSYNC, "NEEDDSYNC" }
+ { VM_FAULT_NEEDDSYNC, "NEEDDSYNC" }, \
+ { VM_FAULT_PTNOTSAME, "PTNOTSAME" }

struct vm_special_mapping {
const char *name; /* The name, e.g. "[vdso]". */
diff --git a/mm/memory.c b/mm/memory.c
index 221ccdf34991..d5bebca47d98 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -2094,21 +2094,29 @@ static inline 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 vm_fault_t 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)
@@ -2714,8 +2722,17 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
int exclusive = 0;
vm_fault_t ret = 0;

- if (!pte_unmap_same(vma->vm_mm, vmf->pmd, vmf->pte, vmf->orig_pte))
+ ret = pte_unmap_same(vmf);
+ if (ret) {
+ /*
+ * If pte != orig_pte, this means another thread did the
+ * swap operation in our back.
+ * So nothing else to do.
+ */
+ if (ret == VM_FAULT_PTNOTSAME)
+ ret = 0;
goto out;
+ }

entry = pte_to_swp_entry(vmf->orig_pte);
if (unlikely(non_swap_entry(entry))) {
--
2.21.0
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