[PATCH] mmap zero-length hugetlb file with PROT_NONE to protect a hugetlb virtual area

Sometimes, applications need below call to be successful although
"/mnt/hugepages/file1" doesn't exist.

fd = open("/mnt/hugepages/file1", O_CREAT|O_RDWR, 0755);
*addr = mmap(NULL, 0x1024*1024*256, PROT_NONE, 0, fd, 0);

As for regular pages (or files), above call does work, but as for huge
pages, above call would fail because hugetlbfs_file_mmap would fail if
(!(vma->vm_flags & VM_WRITE) && len > inode->i_size).

This capability on huge page is useful on ia64 when the process wants to
protect one area on region 4, so other threads couldn't read/write this
area.  A famous JVM (Java Virtual Machine) implementation on IA64 needs the
capability.

Signed-off-by: Zhang Yanmin <yanmin.zhang@intel.com>
Cc: David Gibson <david@gibson.dropbear.id.au>
Cc: Hugh Dickins <hugh@veritas.com>
[ Expand-on-mmap semantics again... this time matching normal fs's. wli ]
Acked-by: William Lee Irwin III <wli@holomorphy.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 1 insertions, 3 deletions

diff --git a/fs/hugetlbfs/inode.c b/fs/hugetlbfs/inode.c
index 6449cb697967..c3920c96dadf 100644
--- a/fs/hugetlbfs/inode.c
+++ b/fs/hugetlbfs/inode.c
@@ -83,8 +83,6 @@ static int hugetlbfs_file_mmap(struct file *file, struct vm_area_struct *vma)
 
         ret = -ENOMEM;
         len = vma_len + ((loff_t)vma->vm_pgoff << PAGE_SHIFT);
-        if (!(vma->vm_flags & VM_WRITE) && len > inode->i_size)
-                goto out;
 
         if (vma->vm_flags & VM_MAYSHARE &&
             hugetlb_reserve_pages(inode, vma->vm_pgoff >> (HPAGE_SHIFT-PAGE_SHIFT),
@@ -93,7 +91,7 @@ static int hugetlbfs_file_mmap(struct file *file, struct vm_area_struct *vma)
 
         ret = 0;
         hugetlb_prefault_arch_hook(vma->vm_mm);
-        if (inode->i_size < len)
+        if (vma->vm_flags & VM_WRITE && inode->i_size < len)
                 inode->i_size = len;
 out:
         mutex_unlock(&inode->i_mutex);