linux-kernel - [PATCH master] Ignore failure to unmap INVALID_GRANT

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Message-Id: <20220708163750.2005-8-demi@invisiblethingslab.com>
Date:   Fri,  8 Jul 2022 12:37:51 -0400
From:   Demi Marie Obenour <demi@...isiblethingslab.com>
To:     Juergen Gross <jgross@...e.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        Stefano Stabellini <sstabellini@...nel.org>,
        Oleksandr Tyshchenko <oleksandr_tyshchenko@...m.com>,
        Demi Marie Obenour <demi@...isiblethingslab.com>
Cc:     Linux kernel regressions <regressions@...ts.linux.dev>,
        stable@...r.kernel.org,
        Linux kernel mailing list <linux-kernel@...r.kernel.org>,
        Xen developer discussion <xen-devel@...ts.xenproject.org>
Subject: [PATCH master] Ignore failure to unmap INVALID_GRANT_HANDLE

The error paths of gntdev_mmap() can call unmap_grant_pages() even
though not all of the pages have been successfully mapped.  This will
trigger the WARN_ON()s in __unmap_grant_pages_done().  The number of
warnings can be very large; I have observed thousands of lines of
warnings in the systemd journal.

Avoid this problem by only warning on unmapping failure if the handle
being unmapped is not INVALID_GRANT_HANDLE.  The handle field of any
page that was not successfully mapped will be INVALID_GRANT_HANDLE, so
this catches all cases where unmapping can legitimately fail.

Suggested-by: Juergen Gross <jgross@...e.com>
Cc: stable@...r.kernel.org
Signed-off-by: Demi Marie Obenour <demi@...isiblethingslab.com>
Fixes: dbe97cff7dd9 ("xen/gntdev: Avoid blocking in unmap_grant_pages()")
---
 drivers/xen/gntdev.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/drivers/xen/gntdev.c b/drivers/xen/gntdev.c
index 4b56c39f766d4da68570d08d963f6ef40c8d9c37..22fcd503f4a4487d0aed147c94f432683dad8c73 100644
--- a/drivers/xen/gntdev.c
+++ b/drivers/xen/gntdev.c
@@ -396,13 +396,17 @@ static void __unmap_grant_pages_done(int result,
 	unsigned int offset = data->unmap_ops - map->unmap_ops;
 
 	for (i = 0; i < data->count; i++) {
-		WARN_ON(map->unmap_ops[offset+i].status);
+		WARN_ON(map->unmap_ops[offset+i].status &&
+			map->unmap_ops[offset+i].handle !=
+			INVALID_GRANT_HANDLE);
 		pr_debug("unmap handle=%d st=%d\n",
 			map->unmap_ops[offset+i].handle,
 			map->unmap_ops[offset+i].status);
 		map->unmap_ops[offset+i].handle = INVALID_GRANT_HANDLE;
 		if (use_ptemod) {
-			WARN_ON(map->kunmap_ops[offset+i].status);
+			WARN_ON(map->kunmap_ops[offset+i].status &&
+				map->kunmap_ops[offset+i].handle !=
+				INVALID_GRANT_HANDLE);
 			pr_debug("kunmap handle=%u st=%d\n",
 				 map->kunmap_ops[offset+i].handle,
 				 map->kunmap_ops[offset+i].status);
-- 
Sincerely,
Demi Marie Obenour (she/her/hers)
Invisible Things Lab