[PATCH] shmem_populate: avoid an useless check, and some comments

Either shmem_getpage returns a failure, or it found a page, or it was told
it couldn't do any I/O.  So it's useless to check nonblock in the else
branch.  We could add a BUG() there but I preferred to comment the
offending function.

This was taken out from one Ingo Molnar's old patch I'm resurrecting.
Signed-off-by: Paolo 'Blaisorblade' Giarrusso <blaisorblade@yahoo.it>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Hugh Dickins <hugh@veritas.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

mm/filemap.c

@@ -1504,8 +1504,12 @@ int filemap_populate(struct vm_area_struct *vma, unsigned long addr,
 		return -EINVAL;
 
 	page = filemap_getpage(file, pgoff, nonblock);
+
+	/* XXX: This is wrong, a filesystem I/O error may have happened. Fix that as
+	 * done in shmem_populate calling shmem_getpage */
 	if (!page && !nonblock)
 		return -ENOMEM;
+
 	if (page) {
 		err = install_page(mm, vma, addr, page, prot);
 		if (err) {
@@ -1513,6 +1517,9 @@ int filemap_populate(struct vm_area_struct *vma, unsigned long addr,
 			return err;
 		}
 	} else {
+		/* No page was found just because we can't read it in now (being
+		 * here implies nonblock != 0), but the page may exist, so set
+		 * the PTE to fault it in later. */
 		err = install_file_pte(mm, vma, addr, pgoff, prot);
 		if (err)
 			return err;

mm/shmem.c

@@ -1195,6 +1195,7 @@ static int shmem_populate(struct vm_area_struct *vma,
 		err = shmem_getpage(inode, pgoff, &page, sgp, NULL);
 		if (err)
 			return err;
+		/* Page may still be null, but only if nonblock was set. */
 		if (page) {
 			mark_page_accessed(page);
 			err = install_page(mm, vma, addr, page, prot);
@@ -1202,7 +1203,10 @@ static int shmem_populate(struct vm_area_struct *vma,
 				page_cache_release(page);
 				return err;
 			}
-		} else if (nonblock) {
+		} else {
+			/* No page was found just because we can't read it in
+			 * now (being here implies nonblock != 0), but the page
+			 * may exist, so set the PTE to fault it in later. */
     			err = install_file_pte(mm, vma, addr, pgoff, prot);
 			if (err)
 	    			return err;