Staring at hugetlb_wp(), one might wonder where all the logic for shared mappings is when stumbling over a write-protected page in a shared mapping. In fact, there is none, and so far we thought we could get away with that because e.g., mprotect() should always do the right thing and map all pages directly writable.

Looks like we were wrong:

-------------------------------------------------------------------------- #include <stdio.h> #include <stdlib.h> #include <string.h> #include <fcntl.h> #include <unistd.h> #include <errno.h> #include <sys/mman.h>

#define HUGETLB_SIZE (2 * 1024 * 1024u)

static void clear_softdirty(void) { int fd = open("/proc/self/clear_refs", O_WRONLY); const char *ctrl = "4"; int ret;

if (fd < 0) { fprintf(stderr, "open(clear_refs) failed\n"); exit(1); } ret = write(fd, ctrl, strlen(ctrl)); if (ret != strlen(ctrl)) { fprintf(stderr, "write(clear_refs) failed\n"); exit(1); } close(fd); }

int main(int argc, char **argv) { char *map; int fd;

fd = open("/dev/hugepages/tmp", O_RDWR | O_CREAT); if (!fd) { fprintf(stderr, "open() failed\n"); return -errno; } if (ftruncate(fd, HUGETLB_SIZE)) { fprintf(stderr, "ftruncate() failed\n"); return -errno; }

map = mmap(NULL, HUGETLB_SIZE, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0); if (map == MAP_FAILED) { fprintf(stderr, "mmap() failed\n"); return -errno; }

*map = 0;

if (mprotect(map, HUGETLB_SIZE, PROT_READ)) { fprintf(stderr, "mmprotect() failed\n"); return -errno; }

clear_softdirty();

if (mprotect(map, HUGETLB_SIZE, PROT_READ|PROT_WRITE)) { fprintf(stderr, "mmprotect() failed\n"); return -errno; }

*map = 0;

return 0; } --------------------------------------------------------------------------

Above test fails with SIGBUS when there is only a single free hugetlb page. # echo 1 > /sys/kernel/mm/hugepages/hugepages-2048kB/nr_hugepages # ./test Bus error (core dumped)

And worse, with sufficient free hugetlb pages it will map an anonymous page into a shared mapping, for example, messing up accounting during unmap and breaking MAP_SHARED semantics: # echo 2 > /sys/kernel/mm/hugepages/hugepages-2048kB/nr_hugepages # ./test # cat /proc/meminfo | grep HugePages_ HugePages_Total: 2 HugePages_Free: 1 HugePages_Rsvd: 18446744073709551615 HugePages_Surp: 0

Reason in this particular case is that vma_wants_writenotify() will return "true", removing VM_SHARED in vma_set_page_prot() to map pages write-protected. Let's teach vma_wants_writenotify() that hugetlb does not support softdirty tracking.

Fixes: 64e455079e1b ("mm: softdirty: enable write notifications on VMAs after VM_SOFTDIRTY cleared") Cc: stable@vger.kernel.org # v3.18+ Signed-off-by: David Hildenbrand david@redhat.com --- mm/mmap.c | 7 +++++-- 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/mm/mmap.c b/mm/mmap.c index c035020d0c89..9d780f415be3 100644 --- a/mm/mmap.c +++ b/mm/mmap.c @@ -1646,8 +1646,11 @@ int vma_wants_writenotify(struct vm_area_struct *vma, pgprot_t vm_page_prot) pgprot_val(vm_pgprot_modify(vm_page_prot, vm_flags))) return 0;

- /* Do we need to track softdirty? */ - if (vma_soft_dirty_enabled(vma)) + /* + * Do we need to track softdirty? hugetlb does not support softdirty + * tracking yet. + */ + if (vma_soft_dirty_enabled(vma) && !is_vm_hugetlb_page(vma)) return 1;

/* Specialty mapping? */