mm/page_alloc: correct return value of populated elements if bulk array is populated

Dave Jones reported the following

This made it into 5.13 final, and completely breaks NFSD for me
(Serving tcp v3 mounts). Existing mounts on clients hang, as do
new mounts from new clients. Rebooting the server back to rc7
everything recovers.

The commit b3b64ebd3822 ("mm/page_alloc: do bulk array bounds check after
checking populated elements") returns the wrong value if the array is
already populated which is interpreted as an allocation failure. Dave
reported this fixes his problem and it also passed a test running dbench
over NFS.

Link: https://lkml.kernel.org/r/[e-mail]
Fixes: b3b64ebd3822 ("mm/page_alloc: do bulk array bounds check after checking populated elements")
Signed-off-by: Mel Gorman <[e-mail]>
Reported-by: Dave Jones <[e-mail]>
Tested-by: Dave Jones <[e-mail]>
Cc: Dan Carpenter <[e-mail]>
Cc: Jesper Dangaard Brouer <[e-mail]>
Cc: Vlastimil Babka <[e-mail]>
Cc: <[e-mail]> [5.13+]
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

mm/page_alloc.c

···
 
 	/* Already populated array? */
 	if (unlikely(page_array && nr_pages - nr_populated == 0))
-		return 0;
+		return nr_populated;
 
 	/* Use the single page allocator for one page. */
 	if (nr_pages - nr_populated == 1)