linux-kernel - [PATCH] ida: do zeroing in ida_pre

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Message-Id: <20180108225634.15340-1-linux@rasmusvillemoes.dk>
Date:   Mon,  8 Jan 2018 23:56:34 +0100
From:   Rasmus Villemoes <linux@...musvillemoes.dk>
To:     Matthew Wilcox <mawilcox@...rosoft.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     Eric Biggers <ebiggers@...gle.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        linux-kernel@...r.kernel.org
Subject: [PATCH] ida: do zeroing in ida_pre_get

As far as I can tell, the only place the per-cpu ida_bitmap is populated
is in ida_pre_get. The pre-allocated element is stolen in two places in
ida_get_new_above, in both cases immediately followed by a memset(0).

Since ida_get_new_above is called with locks held, do the zeroing in
ida_pre_get, or rather let kmalloc() do it. Also, apparently gcc
generates ~44 bytes of code to do a memset(, 0, 128):

$ scripts/bloat-o-meter vmlinux.{0,1}
add/remove: 0/0 grow/shrink: 2/1 up/down: 5/-88 (-83)
Function                                     old     new   delta
ida_pre_get                                  115     119      +4
vermagic                                      27      28      +1
ida_get_new_above                            715     627     -88

Signed-off-by: Rasmus Villemoes <linux@...musvillemoes.dk>
---
 lib/idr.c        | 2 --
 lib/radix-tree.c | 2 +-
 2 files changed, 1 insertion(+), 3 deletions(-)

diff --git a/lib/idr.c b/lib/idr.c
index 2593ce513a18..37c552395b3a 100644
--- a/lib/idr.c
+++ b/lib/idr.c
@@ -300,7 +300,6 @@ int ida_get_new_above(struct ida *ida, int start, int *id)
 			bitmap = this_cpu_xchg(ida_bitmap, NULL);
 			if (!bitmap)
 				return -EAGAIN;
-			memset(bitmap, 0, sizeof(*bitmap));
 			bitmap->bitmap[0] = tmp >> RADIX_TREE_EXCEPTIONAL_SHIFT;
 			rcu_assign_pointer(*slot, bitmap);
 		}
@@ -333,7 +332,6 @@ int ida_get_new_above(struct ida *ida, int start, int *id)
 			bitmap = this_cpu_xchg(ida_bitmap, NULL);
 			if (!bitmap)
 				return -EAGAIN;
-			memset(bitmap, 0, sizeof(*bitmap));
 			__set_bit(bit, bitmap->bitmap);
 			radix_tree_iter_replace(root, &iter, slot, bitmap);
 		}
diff --git a/lib/radix-tree.c b/lib/radix-tree.c
index c8d55565fafa..553248b64d60 100644
--- a/lib/radix-tree.c
+++ b/lib/radix-tree.c
@@ -2124,7 +2124,7 @@ int ida_pre_get(struct ida *ida, gfp_t gfp)
 		preempt_enable();
 
 	if (!this_cpu_read(ida_bitmap)) {
-		struct ida_bitmap *bitmap = kmalloc(sizeof(*bitmap), gfp);
+		struct ida_bitmap *bitmap = kzalloc(sizeof(*bitmap), gfp);
 		if (!bitmap)
 			return 0;
 		if (this_cpu_cmpxchg(ida_bitmap, NULL, bitmap))
-- 
2.15.1