RE: [PATCH net-next] net: mana: Improve mana_set_channels() for low mem conditions

[Haiyang Zhang]

> -----Original Message-----
> From: Gerhard Engleder <gerhard@xxxxxxxxxxxxxxxxxxxxx>
> Sent: Thursday, August 29, 2024 3:54 PM
> To: Shradha Gupta <shradhagupta@xxxxxxxxxxxxxxxxxxx>; linux-
> hyperv@xxxxxxxxxxxxxxx; netdev@xxxxxxxxxxxxxxx; linux-
> kernel@xxxxxxxxxxxxxxx; linux-rdma@xxxxxxxxxxxxxxx
> Cc: KY Srinivasan <kys@xxxxxxxxxxxxx>; Haiyang Zhang
> <haiyangz@xxxxxxxxxxxxx>; Wei Liu <wei.liu@xxxxxxxxxx>; Dexuan Cui
> <decui@xxxxxxxxxxxxx>; David S. Miller <davem@xxxxxxxxxxxxx>; Eric
> Dumazet <edumazet@xxxxxxxxxx>; Jakub Kicinski <kuba@xxxxxxxxxx>; Paolo
> Abeni <pabeni@xxxxxxxxxx>; Long Li <longli@xxxxxxxxxxxxx>; Simon Horman
> <horms@xxxxxxxxxx>; Konstantin Taranov <kotaranov@xxxxxxxxxxxxx>;
> Souradeep Chakrabarti <schakrabarti@xxxxxxxxxxxxxxxxxxx>; Erick Archer
> <erick.archer@xxxxxxxxxxx>; Pavan Chebbi <pavan.chebbi@xxxxxxxxxxxx>;
> Ahmed Zaki <ahmed.zaki@xxxxxxxxx>; Colin Ian King
> <colin.i.king@xxxxxxxxx>; Shradha Gupta <shradhagupta@xxxxxxxxxxxxx>
> Subject: Re: [PATCH net-next] net: mana: Improve mana_set_channels() for
> low mem conditions
> 
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> 
> On 29.08.24 16:16, Shradha Gupta wrote:
> > The mana_set_channels() function requires detaching the mana
> > driver and reattaching it with changed channel values.
> > During this operation if the system is low on memory, the reattach
> > might fail, causing the network device being down.
> > To avoid this we pre-allocate buffers at the beginning of set
> operation,
> > to prevent complete network loss
> >
> > Signed-off-by: Shradha Gupta <shradhagupta@xxxxxxxxxxxxxxxxxxx>
> > ---
> >   .../ethernet/microsoft/mana/mana_ethtool.c    | 28 +++++++++++-------
> -
> >   1 file changed, 16 insertions(+), 12 deletions(-)
> >
> > diff --git a/drivers/net/ethernet/microsoft/mana/mana_ethtool.c
> b/drivers/net/ethernet/microsoft/mana/mana_ethtool.c
> > index d6a35fbda447..5077493fdfde 100644
> > --- a/drivers/net/ethernet/microsoft/mana/mana_ethtool.c
> > +++ b/drivers/net/ethernet/microsoft/mana/mana_ethtool.c
> > @@ -345,27 +345,31 @@ static int mana_set_channels(struct net_device
> *ndev,
> >       struct mana_port_context *apc = netdev_priv(ndev);
> >       unsigned int new_count = channels->combined_count;
> >       unsigned int old_count = apc->num_queues;
> > -     int err, err2;
> > +     int err;
> > +
> > +     apc->num_queues = new_count;
> > +     err = mana_pre_alloc_rxbufs(apc, ndev->mtu);
> > +     apc->num_queues = old_count;
> 
> Are you sure that temporary changing num_queues has no side effects on
> other num_queues users like mana_chn_setxdp()?
> 

mana_chn_setxdp() is protected by rtnl_lock, which is OK. But I'm not sure
if all other users are protected. mana_get_stats64() seems not.

@Shradha Gupta You can add num_queues as an argument of mana_pre_alloc_rxbufs()
to avoid changing apc->num_queues.

Thanks,
- Haiyang