linux-kernel - [PATCH 4/7] vmw_balloon: do not use 2MB without batching

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Date:   Wed, 13 Jun 2018 04:03:56 -0700
From:   Nadav Amit <namit@...are.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
CC:     Xavier Deguillard <xdeguillard@...are.com>,
        <linux-kernel@...r.kernel.org>, Arnd Bergmann <arnd@...db.de>,
        Nadav Amit <namit@...are.com>,
        Nadav Amit <nadav.amit@...il.com>
Subject: [PATCH 4/7] vmw_balloon: do not use 2MB without batching

If the hypervisor sets 2MB batching is on, while batching is cleared,
the balloon code breaks. In this case the legacy mechanism is used with
2MB page. The VM would report a 2MB page is ballooned, and the
hypervisor would only take the first 4KB.

While the hypervisor should not report such settings, make the code more
robust by not enabling 2MB support without batching.

Reviewed-by: Xavier Deguillard <xdeguillard@...are.com>
Signed-off-by: Nadav Amit <nadav.amit@...il.com>
---
 drivers/misc/vmw_balloon.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/drivers/misc/vmw_balloon.c b/drivers/misc/vmw_balloon.c
index 496607a9046a..db3cf91a515d 100644
--- a/drivers/misc/vmw_balloon.c
+++ b/drivers/misc/vmw_balloon.c
@@ -339,7 +339,13 @@ static bool vmballoon_send_start(struct vmballoon *b, unsigned long req_caps)
 		success = false;
 	}
 
-	if (b->capabilities & VMW_BALLOON_BATCHED_2M_CMDS)
+	/*
+	 * 2MB pages are only supported with batching. If batching is for some
+	 * reason disabled, do not use 2MB pages, since otherwise the legacy
+	 * mechanism is used with 2MB pages, causing a failure.
+	 */
+	if ((b->capabilities & VMW_BALLOON_BATCHED_2M_CMDS) &&
+	    (b->capabilities & VMW_BALLOON_BATCHED_CMDS))
 		b->supported_page_sizes = 2;
 	else
 		b->supported_page_sizes = 1;
-- 
2.17.0