The patch titled
Subject: mm/hugetlb: fix hugetlb not supporting softdirty tracking
has been added to the -mm mm-hotfixes-unstable branch. Its filename is
mm-hugetlb-fix-hugetlb-not-supporting-softdirty-tracking.patch
This patch will shortly appear at
https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-hugetlb-fix-hugetlb-not-supporting-softdirty-tracking.patch
This patch will later appear in the mm-hotfixes-unstable branch at
git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
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------------------------------------------------------
From: David Hildenbrand <david@xxxxxxxxxx>
Subject: mm/hugetlb: fix hugetlb not supporting softdirty tracking
Date: Thu, 11 Aug 2022 12:34:34 +0200
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/20220811103435.188481-1-david@xxxxxxxxxx
Link: https://lkml.kernel.org/r/20220811103435.188481-2-david@xxxxxxxxxx
Fixes: 64e455079e1b ("mm: softdirty: enable write notifications on VMAs after VM_SOFTDIRTY cleared")
Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
Reviewed-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
Cc: Peter Feiner <pfeiner@xxxxxxxxxx>
Cc: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
Cc: Cyrill Gorcunov <gorcunov@xxxxxxxxxx>
Cc: Pavel Emelyanov <xemul@xxxxxxxxxxxxx>
Cc: Jamie Liu <jamieliu@xxxxxxxxxx>
Cc: Hugh Dickins <hughd@xxxxxxxxxx>
Cc: Naoya Horiguchi <n-horiguchi@xxxxxxxxxxxxx>
Cc: Bjorn Helgaas <bhelgaas@xxxxxxxxxx>
Cc: Muchun Song <songmuchun@xxxxxxxxxxxxx>
Cc: Peter Xu <peterx@xxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx> [3.18+]
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
mm/mmap.c | 7 +++++--
1 file changed, 5 insertions(+), 2 deletions(-)
--- a/mm/mmap.c~mm-hugetlb-fix-hugetlb-not-supporting-softdirty-tracking
+++ a/mm/mmap.c
@@ -1646,8 +1646,11 @@ int vma_wants_writenotify(struct vm_area
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? */
_
Patches currently in -mm which might be from david@xxxxxxxxxx are
mm-gup-fix-foll_force-cow-security-issue-and-remove-foll_cow.patch
mm-hugetlb-fix-hugetlb-not-supporting-softdirty-tracking.patch
mm-hugetlb-support-write-faults-in-shared-mappings.patch
