mremap: check if it's possible to split original vma

If original VMA can't be split at the desired address, do_munmap() will
fail and leave both new-copied VMA and old VMA.  De-facto it's
MREMAP_DONTUNMAP behaviour, which is unexpected.

Currently, it may fail such way for hugetlbfs and dax device mappings.

Minimize such unpleasant situations to OOM by checking .may_split() before
attempting to create a VMA copy.

Link: https://lkml.kernel.org/r/20201013013416.390574-6-dima@arista.com
Signed-off-by: Dmitry Safonov <dima@arista.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Brian Geffon <bgeffon@google.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Dan Carpenter <dan.carpenter@oracle.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Dave Jiang <dave.jiang@intel.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jason Gunthorpe <jgg@ziepe.ca>
Cc: John Hubbard <jhubbard@nvidia.com>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Ralph Campbell <rcampbell@nvidia.com>
Cc: Russell King <linux@armlinux.org.uk>
Cc: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Vishal Verma <vishal.l.verma@intel.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Will Deacon <will@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/mm/mremap.c b/mm/mremap.c
index 366b3de..c5590af 100644 (file)
--- a/mm/mremap.c
+++ b/mm/mremap.c
@@ -493,7 +493,7 @@ static unsigned long move_vma(struct vm_area_struct *vma,
        unsigned long excess = 0;
        unsigned long hiwater_vm;
        int split = 0;
-       int err;
+       int err = 0;
        bool need_rmap_locks;
 
        /*
@@ -503,6 +503,15 @@ static unsigned long move_vma(struct vm_area_struct *vma,
        if (mm->map_count >= sysctl_max_map_count - 3)
                return -ENOMEM;
 
+       if (vma->vm_ops && vma->vm_ops->may_split) {
+               if (vma->vm_start != old_addr)
+                       err = vma->vm_ops->may_split(vma, old_addr);
+               if (!err && vma->vm_end != old_addr + old_len)
+                       err = vma->vm_ops->may_split(vma, old_addr + old_len);
+               if (err)
+                       return err;
+       }
+
        /*
         * Advise KSM to break any KSM pages in the area to be moved:
         * it would be confusing if they were to turn up at the new