The following test case triggers NULL-pointer derefernce in
try_to_unmap_one():
#include <fcntl.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/mman.h>
int main(int argc, char *argv[])
{
int fd;
system("mount -t tmpfs -o huge=always none /mnt");
fd = open("/mnt/test", O_CREAT | O_RDWR);
ftruncate(fd, 2UL << 20);
mmap(NULL, 2UL << 20, PROT_READ | PROT_WRITE,
MAP_SHARED | MAP_FIXED | MAP_LOCKED, fd, 0);
mmap(NULL, 2UL << 20, PROT_READ | PROT_WRITE,
MAP_SHARED | MAP_LOCKED, fd, 0);
munlockall();
return 0;
}
Apparently, there's a case when we call try_to_unmap() on huge PMDs:
it's TTU_MUNLOCK.
Let's handle this case correctly.
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
Fixes: c7ab0d2fdc84 ("mm: convert try_to_unmap_one() to use page_vma_mapped_walk()")
---
mm/rmap.c | 13 +++++++------
1 file changed, 7 insertions(+), 6 deletions(-)
diff --git a/mm/rmap.c b/mm/rmap.c
index 8774791e2809..48ac9ae9bad0 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1314,12 +1314,6 @@ static int try_to_unmap_one(struct page *page, struct vm_area_struct *vma,
}
while (page_vma_mapped_walk(&pvmw)) {
- subpage = page - page_to_pfn(page) + pte_pfn(*pvmw.pte);
- address = pvmw.address;
-
- /* Unexpected PMD-mapped THP? */
- VM_BUG_ON_PAGE(!pvmw.pte, page);
-
/*
* If the page is mlock()d, we cannot swap it out.
* If it's recently referenced (perhaps page_referenced
@@ -1343,6 +1337,13 @@ static int try_to_unmap_one(struct page *page, struct vm_area_struct *vma,
continue;
}
+ /* Unexpected PMD-mapped THP? */
+ VM_BUG_ON_PAGE(!pvmw.pte, page);
+
+ subpage = page - page_to_pfn(page) + pte_pfn(*pvmw.pte);
+ address = pvmw.address;
+
+
if (!(flags & TTU_IGNORE_ACCESS)) {
if (ptep_clear_flush_young_notify(vma, address,
pvmw.pte)) {
--
2.11.0
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