Regarding Virtual address split for a specific process

Regarding Virtual address split for a specific process

[Yeleswarapu, Nagaradhesh]

[AMD Official Use Only - General]

Hi,

From the config options(CONFIG_VMSPLIT_3G/2G/1G), looks like 3/1 or 2/2 or 1/3 split is for all the user space processes.
Is it possible to do 2/2 split for a specific process and let all the other processes use default 3/1 (user/kernel) VA split?

Thanks,
Nagaradhesh

Re: Regarding Virtual address split for a specific process

[Vlastimil Babka (SUSE)]

On 8/1/22 08:45, Yeleswarapu, Nagaradhesh wrote:
> [AMD Official Use Only - General]
> 
> Hi,
> 
> From the config options(CONFIG_VMSPLIT_3G/2G/1G), looks like 3/1 or 2/2 or 1/3 split is for all the user space processes.

Yeah, but only for 32bit architectures.

> Is it possible to do 2/2 split for a specific process and let all the other processes use default 3/1 (user/kernel) VA split?

No, it's hardcoded in the kernel compilation. What would be the use case?

> Thanks,
> Nagaradhesh

RE: Regarding Virtual address split for a specific process

[Yeleswarapu, Nagaradhesh]

[AMD Official Use Only - General]

> On 8/1/22 08:45, Yeleswarapu, Nagaradhesh wrote:
> > [AMD Official Use Only - General]
> >
> > Hi,
> >
> > From the config options(CONFIG_VMSPLIT_3G/2G/1G), looks like 3/1 or 2/2 or
> 1/3 split is for all the user space processes.
> 
> Yeah, but only for 32bit architectures.
Thanks for your response, can you please share details on how to change memory split for a 64bit architecture?

> 
> > Is it possible to do 2/2 split for a specific process and let all the other
> processes use default 3/1 (user/kernel) VA split?
> 
> No, it's hardcoded in the kernel compilation. What would be the use case?
We are trying to offload a task from one Host to another Host, to achieve this we are looking to change 
offload process Virtual address split to 2G/2G and let other processes use default memory split(3G/1G).

Thanks,
Nagaradhesh

Re: Regarding Virtual address split for a specific process

[Matthew Wilcox]

On Tue, Aug 02, 2022 at 12:08:36PM +0000, Yeleswarapu, Nagaradhesh wrote:
> [AMD Official Use Only - General]
> 
> > On 8/1/22 08:45, Yeleswarapu, Nagaradhesh wrote:
> > > [AMD Official Use Only - General]
> > >
> > > Hi,
> > >
> > > From the config options(CONFIG_VMSPLIT_3G/2G/1G), looks like 3/1 or 2/2 or
> > 1/3 split is for all the user space processes.
> > 
> > Yeah, but only for 32bit architectures.
> Thanks for your response, can you please share details on how to change memory split for a 64bit architecture?

You can't.  With a 64-bit kernel, you either get 47 bits (128TB) of
virtual address for a 64-bit process with 4-level page tables or 56 bits
(64PB) with 64-bit kernels.

I believe 32-bit tasks get almost 4GB of address space when run on a
64-bit kernel, but honestly I've forgotten.

> > 
> > > Is it possible to do 2/2 split for a specific process and let all the other
> > processes use default 3/1 (user/kernel) VA split?
> > 
> > No, it's hardcoded in the kernel compilation. What would be the use case?
> We are trying to offload a task from one Host to another Host, to achieve this we are looking to change 
> offload process Virtual address split to 2G/2G and let other processes use default memory split(3G/1G).

32-bit kernels are still relevant?  They don't even get security fixes
in some cases.
https://lore.kernel.org/lkml/Ys%2FkfDk7mVE09N3L@worktop.programming.kicks-ass.net/