Re: [PATCH net 1/2] net/devmem: Reject insufficiently large dmabuf pools

Mina Almasry
2025-04-24T23:42:37Z

On Thu, Apr 24, 2025 at 3:40 PM Stanislav Fomichev <stfomichev@gmail.com> wrote:
>
> On 04/24, Mina Almasry wrote:
> > On Thu, Apr 24, 2025 at 3:10 PM Stanislav Fomichev <stfomichev@gmail.com> wrote:
> > >
> > > On 04/24, Mina Almasry wrote:
> > > > On Wed, Apr 23, 2025 at 1:15 PM Stanislav Fomichev <stfomichev@gmail.com> wrote:
> > > > >
> > > > > On 04/23, Mina Almasry wrote:
> > > > > > On Wed, Apr 23, 2025 at 9:03 AM Cosmin Ratiu <cratiu@nvidia.com> wrote:
> > > > > > >
> > > > > > > Drivers that are told to allocate RX buffers from pools of DMA memory
> > > > > > > should have enough memory in the pool to satisfy projected allocation
> > > > > > > requests (a function of ring size, MTU & other parameters). If there's
> > > > > > > not enough memory, RX ring refill might fail later at inconvenient times
> > > > > > > (e.g. during NAPI poll).
> > > > > > >
> > > > > >
> > > > > > My understanding is that if the RX ring refill fails, the driver will
> > > > > > post the buffers it was able to allocate data for, and will not post
> > > > > > other buffers. So it will run with a degraded performance but nothing
> > > > > > overly bad should happen. This should be the same behavior if the
> > > > > > machine is under memory pressure.
> > > > > >
> > > > > > In general I don't know about this change. If the user wants to use
> > > > > > very small dmabufs, they should be able to, without going through
> > > > > > hoops reducing the number of rx ring slots the driver has (if it
> > > > > > supports configuring that).
> > > > > >
> > > > > > I think maybe printing an error or warning that the dmabuf is too
> > > > > > small for the pool_size may be fine. But outright failing this
> > > > > > configuration? I don't think so.
> > > > > >
> > > > > > > This commit adds a check at dmabuf pool init time that compares the
> > > > > > > amount of memory in the underlying chunk pool (configured by the user
> > > > > > > space application providing dmabuf memory) with the desired pool size
> > > > > > > (previously set by the driver) and fails with an error message if chunk
> > > > > > > memory isn't enough.
> > > > > > >
> > > > > > > Fixes: 0f9214046893 ("memory-provider: dmabuf devmem memory provider")
> > > > > > > Signed-off-by: Cosmin Ratiu <cratiu@nvidia.com>
> > > > > > > ---
> > > > > > > net/core/devmem.c | 11 +++++++++++
> > > > > > > 1 file changed, 11 insertions(+)
> > > > > > >
> > > > > > > diff --git a/net/core/devmem.c b/net/core/devmem.c
> > > > > > > index 6e27a47d0493..651cd55ebb28 100644
> > > > > > > --- a/net/core/devmem.c
> > > > > > > +++ b/net/core/devmem.c
> > > > > > > @@ -299,6 +299,7 @@ net_devmem_bind_dmabuf(struct net_device *dev, unsigned int dmabuf_fd,
> > > > > > > int mp_dmabuf_devmem_init(struct page_pool *pool)
> > > > > > > {
> > > > > > > struct net_devmem_dmabuf_binding *binding = pool->mp_priv;
> > > > > > > + size_t size;
> > > > > > >
> > > > > > > if (!binding)
> > > > > > > return -EINVAL;
> > > > > > > @@ -312,6 +313,16 @@ int mp_dmabuf_devmem_init(struct page_pool *pool)
> > > > > > > if (pool->p.order != 0)
> > > > > > > return -E2BIG;
> > > > > > >
> > > > > > > + /* Validate that the underlying dmabuf has enough memory to satisfy
> > > > > > > + * requested pool size.
> > > > > > > + */
> > > > > > > + size = gen_pool_size(binding->chunk_pool) >> PAGE_SHIFT;
> > > > > > > + if (size < pool->p.pool_size) {
> > > > > >
> > > > > > pool_size seems to be the number of ptr_ring slots in the page_pool,
> > > > > > not some upper or lower bound on the amount of memory the page_pool
> > > > > > can provide. So this check seems useless? The page_pool can still not
> > > > > > provide this amount of memory with dmabuf (if the netmems aren't being
> > > > > > recycled fast enough) or with normal memory (under memory pressure).
> > > > >
> > > > > I read this check more as "is there enough chunks in the binding to
> > > > > fully fill in the page pool". User controls the size of rx ring
> > > >
> > > > Only on drivers that support ethtool -G, and where it will let you
> > > > configure -G to what you want.
> > >
> > > gve is the minority here, any major nic (brcm/mlx/intel) supports resizing
> > > the rings.
> > >
> >
> > GVE supports resizing rings. Other drivers may not. Even on drivers
> > that support resizing rings. Some users may have a use case for a
> > dmabuf smaller than the minimum ring size their driver accepts.
> >
> > > > > which
> > > > > controls the size of the page pool which somewhat dictates the minimal
> > > > > size of the binding (maybe).
> > > >
> > > > See the test I ran in the other thread. Seems at least GVE is fine
> > > > with dmabuf size < ring size. I don't know what other drivers do, but
> > > > generally speaking I think specific driver limitations should not
> > > > limit what others can do with their drivers. Sure for the GPU mem
> > > > applications you're probably looking at the dmabufs are huge and
> > > > supporting small dmabufs is not a concern, but someone somewhere may
> > > > want to run with 1 MB dmabuf for some use case and if their driver is
> > > > fine with it, core should not prevent it, I think.
> > > >
> > > > > So it's more of a sanity check.
> > > > >
> > > > > Maybe having better defaults in ncdevmem would've been a better option? It
> > > > > allocates (16000*4096) bytes (slightly less than 64MB, why? to fit into
> > > > > default /sys/module/udmabuf/parameters/size_limit_mb?) and on my setup
> > > > > PP wants to get 64MB at least..
> > > >
> > > > Yeah, udmabuf has a limitation that it only supports 64MB max size
> > > > last I looked.
> > >
> > > We can use /sys/module/udmabuf/parameters/size_limit_mb to allocate
> > > more than 64MB, ncdevmem can change it.
> >
> > The udmabuf limit is hardcoded, in udmabuf.c or configured on module
> > load, and ncdevmem doesn't load udmabuf. I guess it could be changed
> > to that, but currently ncdevmem works with CONFIG_UDMABUF=y
>
> You don't need to load/reload the module to change module params:
>
> # id
> uid=0(root) gid=0(root) groups=0(root),1(bin),2(daemon),3(sys)
> # cat /sys/module/udmabuf/parameters/size_limit_mb
> 64
> # echo 128 > /sys/module/udmabuf/parameters/size_limit_mb
> # cat /sys/module/udmabuf/parameters/size_limit_mb
> 128
>

Today I learned! Thanks!

I will put it on my todo list to make ncdevmem force a larger limit to
udmabuf and allocate a larger dmabuf by default. Technically any
dmabuf should be supported, but by default I think probably ncdevmem
should use a .5 GB -> 1GB dmabuf that is more common for these GPU
applications or something. There could be an option as well for folks
to test their driver against smaller dmabufs.

--
Thanks,
Mina