By using smaller datatypes this (rather large) struct shrinks considerably
(80 -> 48 bytes on x86_64).

As this is embedded in other structs, this also rerduces size of several
others, e.g. cls_fl_head or nft_hash.
Signed-off-by: NFlorian Westphal <fw@strlen.de>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

Output to the RDMA driver whether DPM mode is enabled or disabled in
the HW and if so what is the number of WIDs it supports
Signed-off-by: NRam Amrani <Ram.Amrani@cavium.com>
Signed-off-by: NYuval Mintz <Yuval.Mintz@cavium.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

Drivers usually have a number of restrictions for running XDP
- most common being buffer sizes, LRO and number of rings.
Even though some drivers try to be helpful and print error
messages experience shows that users don't often consult
kernel logs on netlink errors.  Try to use the new extended
ack mechanism to carry the message back to user space.
Signed-off-by: NJakub Kicinski <jakub.kicinski@netronome.com>
Acked-by: NDaniel Borkmann <daniel@iogearbox.net>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

As we propagate extended ack reporting throughout various paths in
the kernel it may be that the same function is called with the
extended ack parameter passed as NULL.  One place where that happens
is in drivers which have a centralized reconfiguration function
called both from ndos and from ethtool_ops.  Add a new helper for
setting the error message in such conditions.

Existing helper is left as is to encourage propagating the ext act
fully wherever possible.  It also makes it clear in the code which
messages may be lost due to ext ack being NULL.
Signed-off-by: NJakub Kicinski <jakub.kicinski@netronome.com>
Acked-by: NDaniel Borkmann <daniel@iogearbox.net>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

When IP tunnel encapsulation rules are offloaded, the kernel can't see
the traffic of the offloaded flow. The neighbour for the IP tunnel
destination of the offloaded flow can mistakenly become STALE and
deleted by the kernel since its 'used' value wasn't changed.

To make sure that a neighbour which is used by the HW won't become
STALE, we proactively update the neighbour 'used' value every
DELAY_PROBE_TIME period, when packets were matched and counted by the HW
for one of the tunnel encap flows related to this neighbour.

The periodic task that updates the used neighbours is scheduled when a
tunnel encap rule is successfully offloaded into HW and keeps re-scheduling
itself as long as the representor's neighbours list isn't empty.

Add, remove, lookup and status change operations done over the
representor's neighbours list or the neighbour hash entry encaps list
are all serialized by RTNL lock.
Signed-off-by: NHadar Hen Zion <hadarh@mellanox.com>
Reviewed-by: NOr Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: NSaeed Mahameed <saeedm@mellanox.com>

Parse the BSS max idle period element and set the BSS configuration
accordingly so the driver can use this information to configure the
max idle period and to use protected management frames for keep alive
when required.

The BSS max idle period element is defined in IEEE802.11-2016,
section 9.4.2.79
Signed-off-by: NAvraham Stern <avraham.stern@intel.com>
Signed-off-by: NLuca Coelho <luciano.coelho@intel.com>
Signed-off-by: NJohannes Berg <johannes.berg@intel.com>

Support for l2 multicast flood control was added in commit b6cb5ac8
("net: bridge: add per-port multicast flood flag"). It allows broadcast
as it was introduced specifically for unknown multicast flood control.
But as broadcast is a special case of multicast, this may also need to
be disabled. For this purpose, introduce a flag to disable the flooding
of received l2 broadcasts. This approach is backwards compatible and
provides flexibility in filtering for the desired packet types.

Cc: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Signed-off-by: NMike Manning <mmanning@brocade.com>
Reviewed-by: NNikolay Aleksandrov <nikolay@cumulusnetworks.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

This patch updates the comment for netif_dormant() function to reflect
the intended usage.
Signed-off-by: NZhang Shengju <zhangshengju@cmss.chinamobile.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

When max_size is not set or if it set to a sufficiently large
value, the nelems counter can overflow.  This would cause havoc
with the automatic shrinking as it would then attempt to fit a
huge number of entries into a tiny hash table.

This patch fixes this by adding max_elems to struct rhashtable
to cap the number of elements.  This is set to 2^31 as nelems is
not a precise count.  This is sufficiently smaller than UINT_MAX
that it should be safe.

When max_size is set max_elems will be lowered to at most twice
max_size as is the status quo.
Signed-off-by: NHerbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

Some of the enum definitions are unnamed but there's still
an attempt at documenting them - that doesn't work. Name
them to make that work.
Signed-off-by: NJohannes Berg <johannes.berg@intel.com>

Add the definition for FT-8021.1X AKM selector as defined in
IEEE Std 802.11-2016, table 9-133.
Signed-off-by: NLuca Coelho <luciano.coelho@intel.com>
Signed-off-by: NJohannes Berg <johannes.berg@intel.com>

Add the definitions for SUITE_B and SUITE_B_192 AKM selectors as
defined in IEEE802.11REVmc_D5.0, table 9-132.
Signed-off-by: NLuca Coelho <luciano.coelho@intel.com>
Signed-off-by: NJohannes Berg <johannes.berg@intel.com>

Some devices or distributions use HZ=100 or HZ=250

TCP receive buffer autotuning has poor behavior caused by this choice.
Since autotuning happens after 4 ms or 10 ms, short distance flows
get their receive buffer tuned to a very high value, but after an initial
period where it was frozen to (too small) initial value.

With tp->tcp_mstamp introduction, we can switch to high resolution
timestamps almost for free (at the expense of 8 additional bytes per
TCP structure)

Note that some TCP stacks use usec TCP timestamps where this
patch makes even more sense : Many TCP flows have < 500 usec RTT.
Hopefully this finer TS option can be standardized soon.

Tested:
 HZ=100 kernel
 ./netperf -H lpaa24 -t TCP_RR -l 1000 -- -r 10000,10000 &

 Peer without patch :
 lpaa24:~# ss -tmi dst lpaa23
 ...
 skmem:(r0,rb8388608,...)
 rcv_rtt:10 rcv_space:3210000 minrtt:0.017

 Peer with the patch :
 lpaa23:~# ss -tmi dst lpaa24
 ...
 skmem:(r0,rb428800,...)
 rcv_rtt:0.069 rcv_space:30000 minrtt:0.017

We can see saner RCVBUF, and more precise rcv_rtt information.
Signed-off-by: NEric Dumazet <edumazet@google.com>
Acked-by: NSoheil Hassas Yeganeh <soheil@google.com>
Acked-by: NNeal Cardwell <ncardwell@google.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

We want to use precise timestamps in TCP stack, but we do not
want to call possibly expensive kernel time services too often.

tp->tcp_mstamp is guaranteed to be updated once per incoming packet.

We will use it in the following patches, removing specific
skb_mstamp_get() calls, and removing ack_time from
struct tcp_sacktag_state.
Signed-off-by: NEric Dumazet <edumazet@google.com>
Acked-by: NSoheil Hassas Yeganeh <soheil@google.com>
Acked-by: NNeal Cardwell <ncardwell@google.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

no users in the tree, insecure_max_entries is always set to
ht->p.max_size * 2 in rhtashtable_init().

Replace only spot that uses it with a ht->p.max_size check.
Signed-off-by: NFlorian Westphal <fw@strlen.de>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

The Ethernet link on an interrupt driven PHY was not coming up if the Ethernet
cable was plugged before the Ethernet interface was brought up.

The patch trigger PHY state machine to update link state if PHY was requested to
do auto-negotiation and auto-negotiation complete flag already set.

During power-up cycle the PHY do auto-negotiation, generate interrupt and set
auto-negotiation complete flag. Interrupt is handled by PHY state machine but
doesn't update link state because PHY is in PHY_READY state. After some time
MAC bring up, start and request PHY to do auto-negotiation. If there are no new
settings to advertise genphy_config_aneg() doesn't start PHY auto-negotiation.
PHY continue to stay in auto-negotiation complete state and doesn't fire
interrupt. At the same time PHY state machine expect that PHY started
auto-negotiation and is waiting for interrupt from PHY and it won't get it.

Fixes: 321beec5 ("net: phy: Use interrupts when available in NOLINK state")
Signed-off-by: NAlexander Kochetkov <al.kochet@gmail.com>
Cc: stable <stable@vger.kernel.org> # v4.9+
Tested-by: NRoger Quadros <rogerq@ti.com>
Tested-by: NAlexandre Belloni <alexandre.belloni@free-electrons.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

Similar to ip_register_table, pass nf_hook_ops to ebt_register_table().
This allows to handle hook registration also via pernet_ops and allows
us to avoid use of legacy register_hook api.
Signed-off-by: NFlorian Westphal <fw@strlen.de>
Signed-off-by: NPablo Neira Ayuso <pablo@netfilter.org>

(struct net_device, xdp_prog) field should be moved in RX cache lines,
reducing latencies when a single packet is received on idle host,
since netif_elide_gro() needs it.
Signed-off-by: NEric Dumazet <edumazet@google.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

This provides a generic SKB based non-optimized XDP path which is used
if either the driver lacks a specific XDP implementation, or the user
requests it via a new IFLA_XDP_FLAGS value named XDP_FLAGS_SKB_MODE.

It is arguable that perhaps I should have required something like
this as part of the initial XDP feature merge.

I believe this is critical for two reasons:

1) Accessibility.  More people can play with XDP with less
   dependencies.  Yes I know we have XDP support in virtio_net, but
   that just creates another depedency for learning how to use this
   facility.

   I wrote this to make life easier for the XDP newbies.

2) As a model for what the expected semantics are.  If there is a pure
   generic core implementation, it serves as a semantic example for
   driver folks adding XDP support.

One thing I have not tried to address here is the issue of
XDP_PACKET_HEADROOM, thanks to Daniel for spotting that.  It seems
incredibly expensive to do a skb_cow(skb, XDP_PACKET_HEADROOM) or
whatever even if the XDP program doesn't try to push headers at all.
I think we really need the verifier to somehow propagate whether
certain XDP helpers are used or not.

v5:
 - Handle both negative and positive offset after running prog
 - Fix mac length in XDP_TX case (Alexei)
 - Use rcu_dereference_protected() in free_netdev (kbuild test robot)

v4:
 - Fix MAC header adjustmnet before calling prog (David Ahern)
 - Disable LRO when generic XDP is installed (Michael Chan)
 - Bypass qdisc et al. on XDP_TX and record the event (Alexei)
 - Do not perform generic XDP on reinjected packets (DaveM)

v3:
 - Make sure XDP program sees packet at MAC header, push back MAC
   header if we do XDP_TX.  (Alexei)
 - Elide GRO when generic XDP is in use.  (Alexei)
 - Add XDP_FLAG_SKB_MODE flag which the user can use to request generic
   XDP even if the driver has an XDP implementation.  (Alexei)
 - Report whether SKB mode is in use in rtnl_xdp_fill() via XDP_FLAGS
   attribute.  (Daniel)

v2:
 - Add some "fall through" comments in switch statements based
   upon feedback from Andrew Lunn
 - Use RCU for generic xdp_prog, thanks to Johannes Berg.
Tested-by: NAndy Gospodarek <andy@greyhouse.net>
Tested-by: NJesper Dangaard Brouer <brouer@redhat.com>
Tested-by: NDavid Ahern <dsa@cumulusnetworks.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

This patch adds support for UDP ports in bulletin board
to notify UDP ports change to the VFs
Signed-off-by: NManish Chopra <manish.chopra@cavium.com>
Signed-off-by: NYuval Mintz <yuval.mintz@cavium.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

This patch enables tunnel feature offloads based on hw configuration
at initialization time instead of enabling them always.
Signed-off-by: NManish Chopra <manish.chopra@cavium.com>
Signed-off-by: NYuval Mintz <yuval.mintz@cavium.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

The statistics and its proc output was not implemented as per-net in the
initial network namespace support by Mario Kicherer (8e8cda6d).
This patch adds the missing per-net statistics for the CAN subsystem.
Signed-off-by: NOliver Hartkopp <socketcan@hartkopp.net>
Signed-off-by: NMarc Kleine-Budde <mkl@pengutronix.de>

This patch adds the support of the PCAN-PCI Express FD boards made
by PEAK-System, for computers using the PCI Express slot.

The PCAN-PCI Express FD has one or two CAN FD channels, depending
on the model. A galvanic isolation of the CAN ports protects
the electronics of the card and the respective computer against
disturbances of up to 500 Volts. The PCAN-PCI Express FD can be operated
with ambient temperatures in a range of -40 to +85 °C.

Such boards run an extented version of the CAN-FD IP running into USB
CAN-FD interfaces from PEAK-System, so this patch adds several new commands
and their corresponding data types to the PEAK CAN-FD common definitions
header file too.
Signed-off-by: NStephane Grosjean <s.grosjean@peak-system.com>
Signed-off-by: NMarc Kleine-Budde <mkl@pengutronix.de>

The CAN-FD IP from PEAK-System runs into several kinds of PC CAN-FD
interfaces. Up to now, only the USB CAN-FD adapters were supported by
the Kernel. In order to prepare the adding of some new non-USB CAN-FD
interfaces, this patch moves - and rename - the IP definitions file
from its private (usb) sub-directory into a - newly created - CAN specific
one.
Signed-off-by: NStephane Grosjean <s.grosjean@peak-system.com>
Signed-off-by: NMarc Kleine-Budde <mkl@pengutronix.de>

Now that also the last in-tree user of the xdp_adjust_head bit has
been removed, we can remove the flag from struct bpf_prog altogether.

This, at the same time, also makes sure that any future driver for
XDP comes with bpf_xdp_adjust_head() support right away.

A rejection based on this flag would also mean that tail calls
couldn't be used with such driver as per c2002f98 ("bpf: fix
checking xdp_adjust_head on tail calls") fix, thus lets not allow
for it in the first place.
Signed-off-by: NDaniel Borkmann <daniel@iogearbox.net>
Acked-by: NAlexei Starovoitov <ast@kernel.org>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

Add support for parsing MPLS flows to the flow dissector in preparation for
adding MPLS match support to cls_flower.
Signed-off-by: NBenjamin LaHaise <benjamin.lahaise@netronome.com>
Signed-off-by: NBenjamin LaHaise <bcrl@kvack.org>
Reviewed-by: NJakub Kicinski <jakub.kicinski@netronome.com>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Simon Horman <simon.horman@netronome.com>
Cc: Jamal Hadi Salim <jhs@mojatatu.com>
Cc: Cong Wang <xiyou.wangcong@gmail.com>
Cc: Jiri Pirko <jiri@mellanox.com>
Cc: Eric Dumazet <jhs@mojatatu.com>
Cc: Hadar Hen Zion <hadarh@mellanox.com>
Cc: Gao Feng <fgao@ikuai8.com>
Acked-by: NJiri Pirko <jiri@mellanox.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

Middlebox firewall issues can potentially cause server's data being
blackholed after a successful 3WHS using TFO. Following are the related
reports from Apple:
https://www.nanog.org/sites/default/files/Paasch_Network_Support.pdf
Slide 31 identifies an issue where the client ACK to the server's data
sent during a TFO'd handshake is dropped.
C ---> syn-data ---> S
C <--- syn/ack ----- S
C (accept & write)
C <---- data ------- S
C ----- ACK -> X     S
		[retry and timeout]

https://www.ietf.org/proceedings/94/slides/slides-94-tcpm-13.pdf
Slide 5 shows a similar situation that the server's data gets dropped
after 3WHS.
C ---- syn-data ---> S
C <--- syn/ack ----- S
C ---- ack --------> S
S (accept & write)
C?  X <- data ------ S
		[retry and timeout]

This is the worst failure b/c the client can not detect such behavior to
mitigate the situation (such as disabling TFO). Failing to proceed, the
application (e.g., SSL library) may simply timeout and retry with TFO
again, and the process repeats indefinitely.

The proposed solution is to disable active TFO globally under the
following circumstances:
1. client side TFO socket detects out of order FIN
2. client side TFO socket receives out of order RST

We disable active side TFO globally for 1hr at first. Then if it
happens again, we disable it for 2h, then 4h, 8h, ...
And we reset the timeout to 1hr if a client side TFO sockets not opened
on loopback has successfully received data segs from server.
And we examine this condition during close().

The rational behind it is that when such firewall issue happens,
application running on the client should eventually close the socket as
it is not able to get the data it is expecting. Or application running
on the server should close the socket as it is not able to receive any
response from client.
In both cases, out of order FIN or RST will get received on the client
given that the firewall will not block them as no data are in those
frames.
And we want to disable active TFO globally as it helps if the middle box
is very close to the client and most of the connections are likely to
fail.

Also, add a debug sysctl:
  tcp_fastopen_blackhole_detect_timeout_sec:
    the initial timeout to use when firewall blackhole issue happens.
    This can be set and read.
    When setting it to 0, it means to disable the active disable logic.
Signed-off-by: NWei Wang <weiwan@google.com>
Acked-by: NYuchung Cheng <ycheng@google.com>
Acked-by: NNeal Cardwell <ncardwell@google.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

Some boards [1] leave the PHYs at an invalid state
during system power-up or reset thus causing unreliability
issues with the PHY which manifests as PHY not being detected
or link not functional. To fix this, these PHYs need to be RESET
via a GPIO connected to the PHY's RESET pin.

Some boards have a single GPIO controlling the PHY RESET pin of all
PHYs on the bus whereas some others have separate GPIOs controlling
individual PHY RESETs.

In both cases, the RESET de-assertion cannot be done in the PHY driver
as the PHY will not probe till its reset is de-asserted.
So do the RESET de-assertion in the MDIO bus driver.

[1] - am572x-idk, am571x-idk, a437x-idk
Signed-off-by: NRoger Quadros <rogerq@ti.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

The virtio drivers deal with struct virtio_vsock_pkt.  Add
virtio_transport_deliver_tap_pkt(pkt) for handing packets to the
vsockmon device.

We call virtio_transport_deliver_tap_pkt(pkt) from
net/vmw_vsock/virtio_transport.c and drivers/vhost/vsock.c instead of
common code.  This is because the drivers may drop packets before
handing them to common code - we still want to capture them.
Signed-off-by: NGerard Garcia <ggarcia@deic.uab.cat>
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: NJorgen Hansen <jhansen@vmware.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

The patch adds driver support for static/local dcbx mode. In this mode
adapter brings up the dcbx link with locally configured parameters
instead of performing the dcbx negotiation with the peer. The feature
is useful when peer device/switch doesn't support dcbx.
Signed-off-by: NSudarsana Reddy Kalluru <Sudarsana.Kalluru@cavium.com>
Signed-off-by: NYuval Mintz <Yuval.Mintz@cavium.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

Constants used for tuning are generally a bad idea, especially as hardware
changes over time. Replace the constant 2 jiffies with sysctl variable
netdev_budget_usecs to enable sysadmins to tune the softirq processing.
Also document the variable.

For example, a very fast machine might tune this to 1000 microseconds,
while my regression testing 486DX-25 needs it to be 4000 microseconds on
a nearly idle network to prevent time_squeeze from being incremented.

Version 2: changed jiffies to microseconds for predictable units.
Signed-off-by: NMatthew Whitehead <tedheadster@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

To find out if a netdev is an OVS port.
Signed-off-by: NJiri Pirko <jiri@mellanox.com>
Reviewed-by: NIdo Schimmel <idosch@mellanox.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

commit 83e7e4ce ("mac80211: Use rhltable instead of rhashtable")
removed the last user that made use of 'insecure_elasticity' parameter,
i.e. the default of 16 is used everywhere.

Replace it with a constant.
Signed-off-by: NFlorian Westphal <fw@strlen.de>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

Certain 64-bit systems (e.g. Amlogic Meson GX) require buffers to be
used for DMA to be 8-byte-aligned. struct sdio_func has an embedded
small DMA buffer not meeting this requirement.
When testing switching to descriptor chain mode in meson-gx driver
SDIO is broken therefore. Fix this by allocating the small DMA buffer
separately as kmalloc ensures that the returned memory area is
properly aligned for every basic data type.
Signed-off-by: NHeiner Kallweit <hkallweit1@gmail.com>
Tested-by: NHelmut Klein <hgkr.klein@gmail.com>
Signed-off-by: NUlf Hansson <ulf.hansson@linaro.org>

This patch adds necessary APIs to interface with
qede aRFS support in successive patch.

It also reserves separate PTT entry for aRFS,
[as being in fastpath flow] for hardware access instead of
trying to acquire it at run time from the ptt pool.
Signed-off-by: NManish Chopra <manish.chopra@cavium.com>
Signed-off-by: NYuval Mintz <yuval.mintz@cavium.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

The MTU overhead calculation in L2TP device set-up
merged via commit b784e7eb
needs to be adjusted to lock the tunnel socket while
referencing the sub-data structures to derive the
socket's IP overhead.
Reported-by: NGuillaume Nault <g.nault@alphalink.fr>
Tested-by: NGuillaume Nault <g.nault@alphalink.fr>
Signed-off-by: NR. Parameswaran <rparames@brocade.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

Implement mlx5e's IPoIB SKB transmit using the helper functions provided
by mlx5e ethernet tx flow, the only difference in the code between
mlx5e_xmit and mlx5i_xmit is that IPoIB has some extra fields to fill
(UD datagram segment) in the TX descriptor (WQE) and it doesn't need to
have any vlan handling.
Signed-off-by: NSaeed Mahameed <saeedm@mellanox.com>
Reviewed-by: NErez Shitrit <erezsh@mellanox.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

IB flow tables need the underlay qp to perform flow steering.
Here we change the API of the flow tables creation to accept the
underlay QP number as a parameter in order to support IB (IPoIB) flow
steering.
Signed-off-by: NErez Shitrit <erezsh@mellanox.com>
Signed-off-by: NSaeed Mahameed <saeedm@mellanox.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

New capability bit: ipoib_enhanced_offloads, indicates new ability for UD
QP to do RSS and enhanced IPoIB offloads and acceleration.

Add underlay_qpn to the TIS and flow_table objects In order to support
SET_ROOT command, to connect between IPoIB QPs and flow steering tables.
Signed-off-by: NErez Shitrit <erezsh@mellanox.com>
Signed-off-by: NSaeed Mahameed <saeedm@mellanox.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

Commit 729204ef("block: relax check on sg gap") allows us to merge
bios, if both are physically contiguous.  This change can merge a huge
number of small bios, through mkfs for example, mkfs.ntfs running time
can be decreased to ~1/10.

But if one rq starts with a non-aligned buffer (the 1st bvec's bv_offset
is non-zero) and if we allow the merge, it is quite difficult to respect
sg gap limit, especially the max segment size, or we risk having an
unaligned virtual boundary.  This patch tries to avoid the issue by
disallowing a merge, if the req starts with an unaligned buffer.

Also add comments to explain why the merged segment can't end in
unaligned virt boundary.

Fixes: 729204ef ("block: relax check on sg gap")
Tested-by: NJohannes Thumshirn <jthumshirn@suse.de>
Reviewed-by: NJohannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: NMing Lei <ming.lei@redhat.com>

Rewrote parts of the commit message and comments.
Signed-off-by: NJens Axboe <axboe@fb.com>