[Xen-devel] [PATCH v4] Xen: Fix retry calls into PRIVCMD_MMAPBATCH*.

[Andres Lagar-Cavilla <andreslc@xxxxxxxxxxxxxx>]

From: Andres Lagar-Cavilla <andres@xxxxxxxxxxxxxxxx>

When a foreign mapper attempts to map guest frames that are paged out,
the mapper receives an ENOENT response and will have to try again
while a helper process pages the target frame back in.

Gating checks on PRIVCMD_MMAPBATCH* ioctl args were preventing retries
of mapping calls.

V2: Fixed autotranslated physmap mode breakage introduced by V1.
V3: Enforce full range mapping for first ioctl call on a given VMA.
V4: Shuffle logic to minimize page table walking.

Signed-off-by: Andres Lagar-Cavilla <andres@xxxxxxxxxxxxxxxx>
---
 drivers/xen/privcmd.c |   63 +++++++++++++++++++++++++++++++++++++++++-------
 1 files changed, 53 insertions(+), 10 deletions(-)

diff --git a/drivers/xen/privcmd.c b/drivers/xen/privcmd.c
index f8e5dd7..f1b2462 100644
--- a/drivers/xen/privcmd.c
+++ b/drivers/xen/privcmd.c
@@ -43,9 +43,12 @@ MODULE_LICENSE("GPL");
 
 #define PRIV_VMA_LOCKED ((void *)1)
 
-#ifndef HAVE_ARCH_PRIVCMD_MMAP
 static int privcmd_enforce_singleshot_mapping(struct vm_area_struct *vma);
-#endif
+
+static int privcmd_enforce_singleshot_mapping_granular(
+               struct vm_area_struct *vma,
+               unsigned long addr,
+               unsigned long nr_pages);
 
 static long privcmd_ioctl_hypercall(void __user *udata)
 {
@@ -421,20 +424,34 @@ static long privcmd_ioctl_mmap_batch(void __user *udata, 
int version)
 
        vma = find_vma(mm, m.addr);
        if (!vma ||
-           vma->vm_ops != &privcmd_vm_ops ||
-           (m.addr != vma->vm_start) ||
-           ((m.addr + (nr_pages << PAGE_SHIFT)) != vma->vm_end) ||
-           !privcmd_enforce_singleshot_mapping(vma)) {
-               up_write(&mm->mmap_sem);
+           vma->vm_ops != &privcmd_vm_ops) {
                ret = -EINVAL;
+               up_write(&mm->mmap_sem);
                goto out;
        }
-       if (xen_feature(XENFEAT_auto_translated_physmap)) {
-               ret = alloc_empty_pages(vma, m.num);
-               if (ret < 0) {
+
+       /*
+        * Use singleshot mapping to enforce first-time requirements. Must map 
the
+        * whole VMA range, if ARM or x86-PVH must allocate balloon pages.
+        */
+       if (privcmd_enforce_singleshot_mapping(vma)) {
+               if ((m.addr != vma->vm_start) ||
+                   ((m.addr + (nr_pages << PAGE_SHIFT)) != vma->vm_end) ||
+                   (xen_feature(XENFEAT_auto_translated_physmap) &&
+                    ((ret = alloc_empty_pages(vma, m.num)) < 0))) {
+                       if (ret == 0)
+                               ret = -EINVAL;
                        up_write(&mm->mmap_sem);
                        goto out;
                }
+       } else {
+               if ((m.addr < vma->vm_start) ||
+                   ((m.addr + (nr_pages << PAGE_SHIFT)) > vma->vm_end) ||
+                   !privcmd_enforce_singleshot_mapping_granular(vma, m.addr, 
nr_pages)) {
+                       up_write(&mm->mmap_sem);
+                       ret = -EINVAL;
+                       goto out;
+               }
        }
 
        state.domain        = m.dom;
@@ -540,11 +557,37 @@ static int privcmd_mmap(struct file *file, struct 
vm_area_struct *vma)
        return 0;
 }
 
+/*
+ * For asserting on a whole VMA. This is used by the legacy PRIVCMD_MMAP
+ * call, for MMAPBATCH* to ensure the first ioctl is called on the whole
+ * mapping range, and for autotranslated physmap mode to allocate the ballooned
+ * pages that back a mapping only once.
+ */
 static int privcmd_enforce_singleshot_mapping(struct vm_area_struct *vma)
 {
        return !cmpxchg(&vma->vm_private_data, NULL, PRIV_VMA_LOCKED);
 }
 
+/*
+ * For MMAPBATCH*. This allows asserting the singleshot mapping
+ * on a per pfn/pte basis. Mapping calls that fail with ENOENT
+ * can be then retried until success.
+ */
+static int enforce_singleshot_mapping_fn(pte_t *pte, struct page *pmd_page,
+                       unsigned long addr, void *data)
+{
+       return pte_none(*pte) ? 0 : -EBUSY;
+}
+
+static int privcmd_enforce_singleshot_mapping_granular(
+                  struct vm_area_struct *vma,
+                  unsigned long addr,
+                  unsigned long nr_pages)
+{
+       return apply_to_page_range(vma->vm_mm, addr, nr_pages << PAGE_SHIFT,
+                     enforce_singleshot_mapping_fn, NULL) == 0;
+}
+
 const struct file_operations xen_privcmd_fops = {
        .owner = THIS_MODULE,
        .unlocked_ioctl = privcmd_ioctl,
-- 
1.7.1


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