mm/khugepaged.c: fix khugepaged's request size in collapse_file

collapse_file() in khugepaged passes PAGE_SIZE as the number of pages to
be read to page_cache_sync_readahead().  The intent was probably to read
a single page.  Fix it to use the number of pages to the end of the
window instead.

Fixes: 99cb0db ("mm,thp: add read-only THP support for (non-shmem) FS")
Signed-off-by: David Howells <[email protected]>
Signed-off-by: Matthew Wilcox (Oracle) <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Reviewed-by: Matthew Wilcox (Oracle) <[email protected]>
Acked-by: Song Liu <[email protected]>
Acked-by: Yang Shi <[email protected]>
Acked-by: Pankaj Gupta <[email protected]>
Cc: Eric Biggers <[email protected]>
Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Linus Torvalds <[email protected]>

Author: dhowells

mm/khugepaged.c

@@ -1709,7 +1709,7 @@ static void collapse_file(struct mm_struct *mm,
 				xas_unlock_irq(&xas);
 				page_cache_sync_readahead(mapping, &file->f_ra,
 							  file, index,
-							  PAGE_SIZE);
+							  end - index);
 				/* drain pagevecs to help isolate_lru_page() */
 				lru_add_drain();
 				page = find_lock_page(mapping, index);