mirror of
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git
synced 2024-11-01 17:08:10 +00:00
74d332c13b
Joby Poriyath provided a xen-netback patch to reduce the size of xenvif structure as some netdev allocation could fail under memory pressure/fragmentation. This patch is handling the problem at the core level, allowing any netdev structures to use vmalloc() if kmalloc() failed. As vmalloc() adds overhead on a critical network path, add __GFP_REPEAT to kzalloc() flags to do this fallback only when really needed. Signed-off-by: Eric Dumazet <edumazet@google.com> Reported-by: Joby Poriyath <joby.poriyath@citrix.com> Cc: Ben Hutchings <bhutchings@solarflare.com> Signed-off-by: David S. Miller <davem@davemloft.net>
107 lines
4 KiB
Text
107 lines
4 KiB
Text
|
|
Network Devices, the Kernel, and You!
|
|
|
|
|
|
Introduction
|
|
============
|
|
The following is a random collection of documentation regarding
|
|
network devices.
|
|
|
|
struct net_device allocation rules
|
|
==================================
|
|
Network device structures need to persist even after module is unloaded and
|
|
must be allocated with alloc_netdev_mqs() and friends.
|
|
If device has registered successfully, it will be freed on last use
|
|
by free_netdev(). This is required to handle the pathologic case cleanly
|
|
(example: rmmod mydriver </sys/class/net/myeth/mtu )
|
|
|
|
alloc_netdev_mqs()/alloc_netdev() reserve extra space for driver
|
|
private data which gets freed when the network device is freed. If
|
|
separately allocated data is attached to the network device
|
|
(netdev_priv(dev)) then it is up to the module exit handler to free that.
|
|
|
|
MTU
|
|
===
|
|
Each network device has a Maximum Transfer Unit. The MTU does not
|
|
include any link layer protocol overhead. Upper layer protocols must
|
|
not pass a socket buffer (skb) to a device to transmit with more data
|
|
than the mtu. The MTU does not include link layer header overhead, so
|
|
for example on Ethernet if the standard MTU is 1500 bytes used, the
|
|
actual skb will contain up to 1514 bytes because of the Ethernet
|
|
header. Devices should allow for the 4 byte VLAN header as well.
|
|
|
|
Segmentation Offload (GSO, TSO) is an exception to this rule. The
|
|
upper layer protocol may pass a large socket buffer to the device
|
|
transmit routine, and the device will break that up into separate
|
|
packets based on the current MTU.
|
|
|
|
MTU is symmetrical and applies both to receive and transmit. A device
|
|
must be able to receive at least the maximum size packet allowed by
|
|
the MTU. A network device may use the MTU as mechanism to size receive
|
|
buffers, but the device should allow packets with VLAN header. With
|
|
standard Ethernet mtu of 1500 bytes, the device should allow up to
|
|
1518 byte packets (1500 + 14 header + 4 tag). The device may either:
|
|
drop, truncate, or pass up oversize packets, but dropping oversize
|
|
packets is preferred.
|
|
|
|
|
|
struct net_device synchronization rules
|
|
=======================================
|
|
ndo_open:
|
|
Synchronization: rtnl_lock() semaphore.
|
|
Context: process
|
|
|
|
ndo_stop:
|
|
Synchronization: rtnl_lock() semaphore.
|
|
Context: process
|
|
Note: netif_running() is guaranteed false
|
|
|
|
ndo_do_ioctl:
|
|
Synchronization: rtnl_lock() semaphore.
|
|
Context: process
|
|
|
|
ndo_get_stats:
|
|
Synchronization: dev_base_lock rwlock.
|
|
Context: nominally process, but don't sleep inside an rwlock
|
|
|
|
ndo_start_xmit:
|
|
Synchronization: __netif_tx_lock spinlock.
|
|
|
|
When the driver sets NETIF_F_LLTX in dev->features this will be
|
|
called without holding netif_tx_lock. In this case the driver
|
|
has to lock by itself when needed. It is recommended to use a try lock
|
|
for this and return NETDEV_TX_LOCKED when the spin lock fails.
|
|
The locking there should also properly protect against
|
|
set_rx_mode. Note that the use of NETIF_F_LLTX is deprecated.
|
|
Don't use it for new drivers.
|
|
|
|
Context: Process with BHs disabled or BH (timer),
|
|
will be called with interrupts disabled by netconsole.
|
|
|
|
Return codes:
|
|
o NETDEV_TX_OK everything ok.
|
|
o NETDEV_TX_BUSY Cannot transmit packet, try later
|
|
Usually a bug, means queue start/stop flow control is broken in
|
|
the driver. Note: the driver must NOT put the skb in its DMA ring.
|
|
o NETDEV_TX_LOCKED Locking failed, please retry quickly.
|
|
Only valid when NETIF_F_LLTX is set.
|
|
|
|
ndo_tx_timeout:
|
|
Synchronization: netif_tx_lock spinlock; all TX queues frozen.
|
|
Context: BHs disabled
|
|
Notes: netif_queue_stopped() is guaranteed true
|
|
|
|
ndo_set_rx_mode:
|
|
Synchronization: netif_addr_lock spinlock.
|
|
Context: BHs disabled
|
|
|
|
struct napi_struct synchronization rules
|
|
========================================
|
|
napi->poll:
|
|
Synchronization: NAPI_STATE_SCHED bit in napi->state. Device
|
|
driver's ndo_stop method will invoke napi_disable() on
|
|
all NAPI instances which will do a sleeping poll on the
|
|
NAPI_STATE_SCHED napi->state bit, waiting for all pending
|
|
NAPI activity to cease.
|
|
Context: softirq
|
|
will be called with interrupts disabled by netconsole.
|