xen-netback: fix occasional leak of grant ref mappings under memory pressure [Linux 5.0]

xen-netback: fix occasional leak of grant ref mappings under memory pressure [Linux 5.0]

This Linux kernel change "xen-netback: fix occasional leak of grant ref mappings under memory pressure" is included in the Linux 5.0 release. This change is authored by Igor Druzhinin <igor.druzhinin [at] citrix.com> on Thu Feb 28 12:48:03 2019 +0000. The commit for this change in Linux stable tree is 99e87f5 (patch).

xen-netback: fix occasional leak of grant ref mappings under memory pressure

Zero-copy callback flag is not yet set on frag list skb at the moment
xenvif_handle_frag_list() returns -ENOMEM. This eventually results in
leaking grant ref mappings since xenvif_zerocopy_callback() is never
called for these fragments. Those eventually build up and cause Xen
to kill Dom0 as the slots get reused for new mappings:

"d0v0 Attempt to implicitly unmap a granted PTE c010000329fce005"

That behavior is observed under certain workloads where sudden spikes
of page cache writes coexist with active atomic skb allocations from
network traffic. Additionally, rework the logic to deal with frag_list
deallocation in a single place.

Signed-off-by: Paul Durrant <[email protected]>
Signed-off-by: Igor Druzhinin <[email protected]>
Acked-by: Wei Liu <[email protected]>
Signed-off-by: David S. Miller <[email protected]>

There are 10 lines of Linux source code added/deleted in this change. Code changes to Linux kernel are as follows.

 drivers/net/xen-netback/netback.c | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/drivers/net/xen-netback/netback.c b/drivers/net/xen-netback/netback.c
index 80aae3a..f09948b 100644
--- a/drivers/net/xen-netback/netback.c
+++ b/drivers/net/xen-netback/netback.c
@@ -1072,11 +1072,6 @@ static int xenvif_handle_frag_list(struct xenvif_queue *queue, struct sk_buff *s
        skb_frag_size_set(&frags[i], len);
    }

-   /* Copied all the bits from the frag list -- free it. */
-   skb_frag_list_init(skb);
-   xenvif_skb_zerocopy_prepare(queue, nskb);
-   kfree_skb(nskb);
-
    /* Release all the original (foreign) frags. */
    for (f = 0; f < skb_shinfo(skb)->nr_frags; f++)
        skb_frag_unref(skb, f);
@@ -1145,6 +1140,8 @@ static int xenvif_tx_submit(struct xenvif_queue *queue)
        xenvif_fill_frags(queue, skb);

        if (unlikely(skb_has_frag_list(skb))) {
+           struct sk_buff *nskb = skb_shinfo(skb)->frag_list;
+           xenvif_skb_zerocopy_prepare(queue, nskb);
            if (xenvif_handle_frag_list(queue, skb)) {
                if (net_ratelimit())
                    netdev_err(queue->vif->dev,
@@ -1153,6 +1150,9 @@ static int xenvif_tx_submit(struct xenvif_queue *queue)
                kfree_skb(skb);
                continue;
            }
+           /* Copied all the bits from the frag list -- free it. */
+           skb_frag_list_init(skb);
+           kfree_skb(nskb);
        }

        skb->dev      = queue->vif->dev;

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