[Xen-devel] [PATCH] mmap_vmcore: skip non-ram pages reported by hypervisors

[Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>]

we have a special check in read_vmcore() handler to check if the page was
reported as ram or not by the hypervisor (pfn_is_ram()). However, when
vmcore is read with mmap() no such check is performed. That can lead to
unpredictable results, e.g. when running Xen PVHVM guest memcpy() after
mmap() on /proc/vmcore will hang processing HVMMEM_mmio_dm pages creating
enormous load in both DomU and Dom0.

Fix the issue by mapping each non-ram page to the zero page. Keep direct
path with remap_oldmem_pfn_range() to avoid looping through all pages on
bare metal.

The issue can also be solved by overriding remap_oldmem_pfn_range() in
xen-specific code, as remap_oldmem_pfn_range() was been designed for.
That, however, would involve non-obvious xen code path for all x86 builds
with CONFIG_XEN_PVHVM=y and would prevent all other hypervisor-specific
code on x86 arch from doing the same override.

Signed-off-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
---
 fs/proc/vmcore.c | 68 +++++++++++++++++++++++++++++++++++++++++++++++++++-----
 1 file changed, 62 insertions(+), 6 deletions(-)

diff --git a/fs/proc/vmcore.c b/fs/proc/vmcore.c
index 382aa89..2716e19 100644
--- a/fs/proc/vmcore.c
+++ b/fs/proc/vmcore.c
@@ -328,6 +328,46 @@ static inline char *alloc_elfnotes_buf(size_t notes_sz)
  * virtually contiguous user-space in ELF layout.
  */
 #ifdef CONFIG_MMU
+static u64 remap_oldmem_pfn_checked(struct vm_area_struct *vma, u64 len,
+                                   unsigned long pfn, unsigned long page_count)
+{
+       unsigned long pos;
+       size_t size;
+       unsigned long vma_addr;
+       unsigned long emptypage_pfn = __pa(empty_zero_page) >> PAGE_SHIFT;
+
+       for (pos = pfn; (pos - pfn) <= page_count; pos++) {
+               if (!pfn_is_ram(pos) || (pos - pfn) == page_count) {
+                       /* we hit a page which is not ram or reached the end */
+                       if (pos - pfn > 0) {
+                               /* remapping continuous region */
+                               size = (pos - pfn) << PAGE_SHIFT;
+                               vma_addr = vma->vm_start + len;
+                               if (remap_oldmem_pfn_range(vma, vma_addr,
+                                                          pfn, size,
+                                                          vma->vm_page_prot))
+                                       return len;
+                               len += size;
+                               page_count -= (pos - pfn);
+                       }
+                       if (page_count > 0) {
+                               /* we hit a page which is not ram, replacing
+                                  with an empty one */
+                               vma_addr = vma->vm_start + len;
+                               if (remap_oldmem_pfn_range(vma, vma_addr,
+                                                          emptypage_pfn,
+                                                          PAGE_SIZE,
+                                                          vma->vm_page_prot))
+                                       return len;
+                               len += PAGE_SIZE;
+                               pfn = pos + 1;
+                               page_count--;
+                       }
+               }
+       }
+       return len;
+}
+
 static int mmap_vmcore(struct file *file, struct vm_area_struct *vma)
 {
        size_t size = vma->vm_end - vma->vm_start;
@@ -383,17 +423,33 @@ static int mmap_vmcore(struct file *file, struct 
vm_area_struct *vma)
 
        list_for_each_entry(m, &vmcore_list, list) {
                if (start < m->offset + m->size) {
-                       u64 paddr = 0;
+                       u64 paddr = 0, original_len;
+                       unsigned long pfn, page_count;
 
                        tsz = min_t(size_t, m->offset + m->size - start, size);
                        paddr = m->paddr + start - m->offset;
-                       if (remap_oldmem_pfn_range(vma, vma->vm_start + len,
-                                                  paddr >> PAGE_SHIFT, tsz,
-                                                  vma->vm_page_prot))
-                               goto fail;
+
+                       /* check if oldmem_pfn_is_ram was registered to avoid
+                          looping over all pages without a reason */
+                       if (oldmem_pfn_is_ram) {
+                               pfn = paddr >> PAGE_SHIFT;
+                               page_count = tsz >> PAGE_SHIFT;
+                               original_len = len;
+                               len = remap_oldmem_pfn_checked(vma, len, pfn,
+                                                              page_count);
+                               if (len != original_len + tsz)
+                                       goto fail;
+                       } else {
+                               if (remap_oldmem_pfn_range(vma,
+                                                          vma->vm_start + len,
+                                                          paddr >> PAGE_SHIFT,
+                                                          tsz,
+                                                          vma->vm_page_prot))
+                                       goto fail;
+                               len += tsz;
+                       }
                        size -= tsz;
                        start += tsz;
-                       len += tsz;
 
                        if (size == 0)
                                return 0;
-- 
1.9.3


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