Messages in this thread | | | Date | Wed, 18 Apr 2018 20:55:13 +0300 | From | "Michael S. Tsirkin" <> | Subject | Re: [PATCH] net: don't use kvzalloc for DMA memory |
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On Wed, Apr 18, 2018 at 09:05:54AM -0700, Eric Dumazet wrote: > > > On 04/18/2018 07:34 AM, Mikulas Patocka wrote: > > The patch 74d332c13b21 changes alloc_netdev_mqs to use vzalloc if kzalloc > > fails (later patches change it to kvzalloc). > > > > The problem with this is that if the vzalloc function is actually used, > > virtio_net doesn't work (because it expects that the extra memory should > > be accessible with DMA-API and memory allocated with vzalloc isn't). > > > > This patch changes it back to kzalloc and adds a warning if the allocated > > size is too large (the allocation is unreliable in this case). > > > > Signed-off-by: Mikulas Patocka <mpatocka@redhat.com> > > Fixes: 74d332c13b21 ("net: extend net_device allocation to vmalloc()") > > > > --- > > net/core/dev.c | 3 ++- > > 1 file changed, 2 insertions(+), 1 deletion(-) > > > > Index: linux-2.6/net/core/dev.c > > =================================================================== > > --- linux-2.6.orig/net/core/dev.c 2018-04-16 21:08:36.000000000 +0200 > > +++ linux-2.6/net/core/dev.c 2018-04-18 16:24:43.000000000 +0200 > > @@ -8366,7 +8366,8 @@ struct net_device *alloc_netdev_mqs(int > > /* ensure 32-byte alignment of whole construct */ > > alloc_size += NETDEV_ALIGN - 1; > > > > - p = kvzalloc(alloc_size, GFP_KERNEL | __GFP_RETRY_MAYFAIL); > > + WARN_ON(alloc_size > PAGE_SIZE << PAGE_ALLOC_COSTLY_ORDER); > > + p = kzalloc(alloc_size, GFP_KERNEL | __GFP_RETRY_MAYFAIL); > > if (!p) > > return NULL; > > > > > > Since when a net_device needs to be in DMA zone ???
It's likely that we are not the only device like this. It would be better to find a way to find devices like this.
Imagine you want to pass some data to card. Natural thing is to just put it in a variable and start DMA. However DMA API disallows stack access nowdays, so it's natural to put this within struct device.
See e.g.
commit a725ee3e44e39dab1ec82cc745899a785d2a555e Author: Andy Lutomirski <luto@kernel.org> Date: Mon Jul 18 15:34:49 2016 -0700
virtio-net: Remove more stack DMA
> I would rather fix virtio_net, this looks very suspect to me.
It's been done for years. I'm fine with changing virtio-net and allocating DMA memory separately but I am not sure it's appropriate on net.
And OTOH, shouldn't drivers avoid allocating such huge device structs? Abusing vmalloc won't work well on 32 bit platforms.
> Each virtio_net should probably allocate the exact amount of DMA-memory it wants, > instead of expecting core networking stack to have a huge chunk of DMA-memory for everything.
It's not a DMA memory at all (not a synchronous memory) and it is not huge. It's a small chunk of regular memory that is mapped for DMA for a short while, then unmapped.
-- MST
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