HMM hmm_range parameter usage

HMM hmm_range parameter usage

[Bharat Kumar Gogada]

Hi All,

In 5.4 kernel hmm_range structure has pfn_shift parameter. 
This parameter is removed in the latest kernel.

Can any one help me understand usage of this parameter ?

In 5.4 kernel, if this parameter set to PAGE_SHIFT, does it mean HMM framework 
expects user buffers to be 4K aligned, i.e which are allocated via mmap/aligned_alloc ?

Does HMM have any alignment requirements in 5.4 or current kernel ? Will it work 
with buffers allocated via malloc ?

Regards,
bharat 

 

Re: HMM hmm_range parameter usage

[Alistair Popple]

On Monday, 6 June 2022 10:27:20 PM AEST Bharat Kumar Gogada wrote:
> Hi All,
> 
> In 5.4 kernel hmm_range structure has pfn_shift parameter. 
> This parameter is removed in the latest kernel.
> 
> Can any one help me understand usage of this parameter ?

I think originally it was intended to help hmm_range_fault() format PFNs into 
some kind of HW specific value, but it turned out not to be very useful - see 
2733ea144dcc ("2733ea144dcce789de20988c1056e228a07b1bff") for more background. 
I think just setting it to PAGE_SHIFT would be fine.

> In 5.4 kernel, if this parameter set to PAGE_SHIFT, does it mean HMM 
framework 
> expects user buffers to be 4K aligned, i.e which are allocated via mmap/
aligned_alloc ?

It doesn't matter what the pfn_shift parameter is set to as HMM just copies 
(and reformats) CPU page tables so minimum alignment would be PAGE_SIZE (ie. 
generally 4K). HMM doesn't add any extra alignment constraints.

> Does HMM have any alignment requirements in 5.4 or current kernel ? Will it 
work 
> with buffers allocated via malloc ?

Yes. It should work with any normal page with present PTEs although I haven't 
personally tested it with v5.4 in a long time. There's been a number of 
changes in this area so I'd recommend trying to use a more recent kernel if 
possible.

 - Alistair

> Regards,
> bharat 
> 
>  
> 
> 
>