mm/hugetlb: fix hugetlb not supporting softdirty tracking

Patch series "mm/hugetlb: fix write-fault handling for shared mappings", v2.

I observed that hugetlb does not support/expect write-faults in shared
mappings that would have to map the R/O-mapped page writable -- and I
found two case where we could currently get such faults and would
erroneously map an anon page into a shared mapping.

Reproducers part of the patches.

I propose to backport both fixes to stable trees.  The first fix needs a
small adjustment.


This patch (of 2):

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.

Link: https://lkml.kernel.org/r/[email protected]
Link: https://lkml.kernel.org/r/[email protected]
Fixes: 64e4550 ("mm: softdirty: enable write notifications on VMAs after VM_SOFTDIRTY cleared")
Signed-off-by: David Hildenbrand <[email protected]>
Reviewed-by: Mike Kravetz <[email protected]>
Cc: Peter Feiner <[email protected]>
Cc: Kirill A. Shutemov <[email protected]>
Cc: Cyrill Gorcunov <[email protected]>
Cc: Pavel Emelyanov <[email protected]>
Cc: Jamie Liu <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: Naoya Horiguchi <[email protected]>
Cc: Bjorn Helgaas <[email protected]>
Cc: Muchun Song <[email protected]>
Cc: Peter Xu <[email protected]>
Cc: <[email protected]>	[3.18+]
Signed-off-by: Andrew Morton <[email protected]>

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? */