linux-kernel - [PATCH] bitmap: fix a unproper remap when mpol_rebind

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Message-Id: <20220613112937.65428-1-wang.yi59@zte.com.cn>
Date:   Mon, 13 Jun 2022 19:29:37 +0800
From:   Yi Wang <wang.yi59@....com.cn>
To:     yury.norov@...il.com, andriy.shevchenko@...ux.intel.com,
        linux@...musvillemoes.dk
Cc:     linux-kernel@...r.kernel.org, xue.zhihong@....com.cn,
        wang.yi59@....com.cn, wang.liang82@....com.cn
Subject: [PATCH] bitmap: fix a unproper remap when mpol_rebind_nodemask()

Consider one situation:

The app have two vmas which mbind() to node 1 and node3 respectively,
and its cpuset.mems is 0-3, now set its cpuset.mems to 1,3, according
to current bitmap_remap(), we got:

    1 => 3
    3 => 3

This maybe confused because node 1,3 have already in the new settiing
region but both nodes are binded to the same node 3 now.

Actually we found the situation on a very old libvirt and qemu, but
this can be easily reproduced in the current kernel, so we try to fix
it.

A possible fix way is to ignore the bits in @src have already existed
in @new.

Signed-off-by: Yi Wang <wang.yi59@....com.cn>
---
 lib/bitmap.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/lib/bitmap.c b/lib/bitmap.c
index b18e31ea6e66..b77bf1b3852e 100644
--- a/lib/bitmap.c
+++ b/lib/bitmap.c
@@ -1006,8 +1006,8 @@ unsigned int bitmap_ord_to_pos(const unsigned long *buf, unsigned int ord, unsig
  * @dst point to the same location, then this routine copies @src
  * to @dst.
  *
- * The positions of unset bits in @old are mapped to themselves
- * (the identify map).
+ * The positions of unset bits in @old or bits in @src have already
+ * existed in @new are mapped to themselves (the identify map).
  *
  * Apply the above specified mapping to @src, placing the result in
  * @dst, clearing any bits previously set in @dst.
@@ -1033,7 +1033,7 @@ void bitmap_remap(unsigned long *dst, const unsigned long *src,
 	for_each_set_bit(oldbit, src, nbits) {
 		int n = bitmap_pos_to_ord(old, oldbit, nbits);
 
-		if (n < 0 || w == 0)
+		if (n < 0 || w == 0 || test_bit(oldbit, new))
 			set_bit(oldbit, dst);	/* identity map */
 		else
 			set_bit(bitmap_ord_to_pos(new, n % w, nbits), dst);
-- 
2.33.0.rc0.dirty