From 6e43fc04a6bc357d260583b8440882f28069207f Mon Sep 17 00:00:00 2001 From: David Vrabel Date: Wed, 11 Sep 2013 14:52:48 +0100 Subject: [PATCH] xen-netback: count number required slots for an skb more carefully When a VM is providing an iSCSI target and the LUN is used by the backend domain, the generated skbs for direct I/O writes to the disk have large, multi-page skb->data but no frags. With some lengths and starting offsets, xen_netbk_count_skb_slots() would be one short because the simple calculation of DIV_ROUND_UP(skb_headlen(), PAGE_SIZE) was not accounting for the decisions made by start_new_rx_buffer() which does not guarantee responses are fully packed. For example, a skb with length < 2 pages but which spans 3 pages would be counted as requiring 2 slots but would actually use 3 slots. skb->data: | 1111|222222222222|3333 | Fully packed, this would need 2 slots: |111122222222|22223333 | But because the 2nd page wholy fits into a slot it is not split across slots and goes into a slot of its own: |1111 |222222222222|3333 | Miscounting the number of slots means netback may push more responses than the number of available requests. This will cause the frontend to get very confused and report "Too many frags/slots". The frontend never recovers and will eventually BUG. Fix this by counting the number of required slots more carefully. In xen_netbk_count_skb_slots(), more closely follow the algorithm used by xen_netbk_gop_skb() by introducing xen_netbk_count_frag_slots() which is the dry-run equivalent of netbk_gop_frag_copy(). Signed-off-by: David Vrabel Acked-by: Ian Campbell Signed-off-by: David S. Miller --- drivers/net/xen-netback/netback.c | 94 +++++++++++++++++++++---------- 1 file changed, 64 insertions(+), 30 deletions(-) diff --git a/drivers/net/xen-netback/netback.c b/drivers/net/xen-netback/netback.c index 956130c70036..f3e591c611de 100644 --- a/drivers/net/xen-netback/netback.c +++ b/drivers/net/xen-netback/netback.c @@ -212,6 +212,49 @@ static bool start_new_rx_buffer(int offset, unsigned long size, int head) return false; } +struct xenvif_count_slot_state { + unsigned long copy_off; + bool head; +}; + +unsigned int xenvif_count_frag_slots(struct xenvif *vif, + unsigned long offset, unsigned long size, + struct xenvif_count_slot_state *state) +{ + unsigned count = 0; + + offset &= ~PAGE_MASK; + + while (size > 0) { + unsigned long bytes; + + bytes = PAGE_SIZE - offset; + + if (bytes > size) + bytes = size; + + if (start_new_rx_buffer(state->copy_off, bytes, state->head)) { + count++; + state->copy_off = 0; + } + + if (state->copy_off + bytes > MAX_BUFFER_OFFSET) + bytes = MAX_BUFFER_OFFSET - state->copy_off; + + state->copy_off += bytes; + + offset += bytes; + size -= bytes; + + if (offset == PAGE_SIZE) + offset = 0; + + state->head = false; + } + + return count; +} + /* * Figure out how many ring slots we're going to need to send @skb to * the guest. This function is essentially a dry run of @@ -219,48 +262,39 @@ static bool start_new_rx_buffer(int offset, unsigned long size, int head) */ unsigned int xenvif_count_skb_slots(struct xenvif *vif, struct sk_buff *skb) { + struct xenvif_count_slot_state state; unsigned int count; - int i, copy_off; + unsigned char *data; + unsigned i; - count = DIV_ROUND_UP(skb_headlen(skb), PAGE_SIZE); + state.head = true; + state.copy_off = 0; - copy_off = skb_headlen(skb) % PAGE_SIZE; + /* Slot for the first (partial) page of data. */ + count = 1; + /* Need a slot for the GSO prefix for GSO extra data? */ if (skb_shinfo(skb)->gso_size) count++; - for (i = 0; i < skb_shinfo(skb)->nr_frags; i++) { - unsigned long size = skb_frag_size(&skb_shinfo(skb)->frags[i]); - unsigned long offset = skb_shinfo(skb)->frags[i].page_offset; - unsigned long bytes; - - offset &= ~PAGE_MASK; - - while (size > 0) { - BUG_ON(offset >= PAGE_SIZE); - BUG_ON(copy_off > MAX_BUFFER_OFFSET); - - bytes = PAGE_SIZE - offset; - - if (bytes > size) - bytes = size; + data = skb->data; + while (data < skb_tail_pointer(skb)) { + unsigned long offset = offset_in_page(data); + unsigned long size = PAGE_SIZE - offset; - if (start_new_rx_buffer(copy_off, bytes, 0)) { - count++; - copy_off = 0; - } + if (data + size > skb_tail_pointer(skb)) + size = skb_tail_pointer(skb) - data; - if (copy_off + bytes > MAX_BUFFER_OFFSET) - bytes = MAX_BUFFER_OFFSET - copy_off; + count += xenvif_count_frag_slots(vif, offset, size, &state); - copy_off += bytes; + data += size; + } - offset += bytes; - size -= bytes; + for (i = 0; i < skb_shinfo(skb)->nr_frags; i++) { + unsigned long size = skb_frag_size(&skb_shinfo(skb)->frags[i]); + unsigned long offset = skb_shinfo(skb)->frags[i].page_offset; - if (offset == PAGE_SIZE) - offset = 0; - } + count += xenvif_count_frag_slots(vif, offset, size, &state); } return count; } -- GitLab