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

[Vitaly Kuznetsov <vkuznets@redhat.com>]

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@redhat.com>
---
 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