tmpfs/ramfs: fix VM_MAYSHARE mappings for NOMMU

The nommu do_mmap expects f_op->get_unmapped_area to either succeed or
return -ENOSYS for VM_MAYSHARE (e.g. private read-only) mappings.
Returning addr in the non-MAP_SHARED case was completely wrong, and only
happened to work because addr was 0. However, it prevented VM_MAYSHARE
mappings from sharing backing with the fs cache, and forced such
mappings (including shareable program text) to be copied whenever the
number of mappings transitioned from 0 to 1, impacting performance and
memory usage. Subsequent mappings beyond the first still correctly
shared memory with the first.

Instead, treat VM_MAYSHARE identically to VM_SHARED at the file ops level;
do_mmap already handles the semantic differences between them.

Signed-off-by: Rich Felker <[e-mail]>
Cc: Michal Hocko <[e-mail]>
Cc: Greg Ungerer <[e-mail]>
Cc: Geert Uytterhoeven <[e-mail]>
Cc: Yoshinori Sato <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

fs/ramfs/file-nommu.c

···
 	struct page **pages = NULL, **ptr, *page;
 	loff_t isize;
 
-	if (!(flags & MAP_SHARED))
-		return addr;
-
 	/* the mapping mustn't extend beyond the EOF */
 	lpages = (len + PAGE_SIZE - 1) >> PAGE_SHIFT;
 	isize = i_size_read(inode);
 
-	ret = -EINVAL;
+	ret = -ENOSYS;
 	maxpages = (isize + PAGE_SIZE - 1) >> PAGE_SHIFT;
 	if (pgoff >= maxpages)
 		goto out;
···
 		goto out;
 
 	/* gang-find the pages */
-	ret = -ENOMEM;
 	pages = kcalloc(lpages, sizeof(struct page *), GFP_KERNEL);
 	if (!pages)
 		goto out_free;
···
  */
 static int ramfs_nommu_mmap(struct file *file, struct vm_area_struct *vma)
 {
-	if (!(vma->vm_flags & VM_SHARED))
+	if (!(vma->vm_flags & (VM_SHARED | VM_MAYSHARE)))
 		return -ENOSYS;
 
 	file_accessed(file);