mm/madvise.c: fix madvise() infinite loop under special circumstances

MADVISE_WILLNEED has always been a noop for DAX (formerly XIP) mappings.
Unfortunately madvise_willneed() doesn't communicate this information
properly to the generic madvise syscall implementation.  The calling
convention is quite subtle there.  madvise_vma() is supposed to either
return an error or update &prev otherwise the main loop will never
advance to the next vma and it will keep looping for ever without a way
to get out of the kernel.

It seems this has been broken since introduction.  Nobody has noticed
because nobody seems to be using MADVISE_WILLNEED on these DAX mappings.

[[email protected]: rewrite changelog]
Link: http://lkml.kernel.org/r/[email protected]
Fixes: fe77ba6 ("[PATCH] xip: madvice/fadvice: execute in place")
Signed-off-by: chenjie <[email protected]>
Signed-off-by: guoxuenan <[email protected]>
Acked-by: Michal Hocko <[email protected]>
Cc: Minchan Kim <[email protected]>
Cc: zhangyi (F) <[email protected]>
Cc: Miao Xie <[email protected]>
Cc: Mike Rapoport <[email protected]>
Cc: Shaohua Li <[email protected]>
Cc: Andrea Arcangeli <[email protected]>
Cc: Mel Gorman <[email protected]>
Cc: Kirill A. Shutemov <[email protected]>
Cc: David Rientjes <[email protected]>
Cc: Anshuman Khandual <[email protected]>
Cc: Rik van Riel <[email protected]>
Cc: Carsten Otte <[email protected]>
Cc: Dan Williams <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

mm/madvise.c

@@ -276,15 +276,14 @@ static long madvise_willneed(struct vm_area_struct *vma,
 {
 	struct file *file = vma->vm_file;
 
+	*prev = vma;
 #ifdef CONFIG_SWAP
 	if (!file) {
-		*prev = vma;
 		force_swapin_readahead(vma, start, end);
 		return 0;
 	}
 
 	if (shmem_mapping(file->f_mapping)) {
-		*prev = vma;
 		force_shm_swapin_readahead(vma, start, end,
 					file->f_mapping);
 		return 0;
@@ -299,7 +298,6 @@ static long madvise_willneed(struct vm_area_struct *vma,
 		return 0;
 	}
 
-	*prev = vma;
 	start = ((start - vma->vm_start) >> PAGE_SHIFT) + vma->vm_pgoff;
 	if (end > vma->vm_end)
 		end = vma->vm_end;