Re: [PATCH 3/4] arm64: mm: support large block mapping when rodata=full

[Yang Shi]

On 7/23/25 10:38 AM, Dev Jain wrote:
> On 23/06/25 6:56 pm, Ryan Roberts wrote:
> > [...]
> >
> > > > +
> > > > +int split_leaf_mapping(unsigned long addr)
> > > Thanks for coming up with the code. It does help to understand your 
> > > idea. Now I
> > > see why you suggested "split_mapping(start); split_mapping(end);" 
> > > model. It does
> > > make the implementation easier because we don't need a loop anymore. 
> > > But this
> > > may have a couple of problems:
> > >    1. We need walk the page table twice instead of once. It sounds 
> > > expensive.
> > Yes we need to walk twice. That may be more expensive or less expensive,
> > depending on the size of the range that you are splitting. If the 
> > range is large
> > then your approach loops through every leaf mapping between the start 
> > and end
> > which will be more expensive than just doing 2 walks. If the range is 
> > small then
> > your approach can avoid the second walk, but at the expense of all 
> > the extra
> > loop overhead.
> >
> > My suggestion requires 5 loads (assuming the maximum of 5 levels of 
> > lookup).
> > Personally I think this is probably acceptable? Perhaps we need some 
> > other
> > voices here.
>
> Hello all,
>
> I am starting to implement vmalloc-huge by default with BBML2 no-abort 
> on arm64.
> I see that there is some disagreement related to the way the splitting 
> needs to
> be implemented - I skimmed through the discussions and it will require 
> some work
> to understand what is going on :) hopefully I'll be back soon to give 
> some of
> my opinions.

Hi Dev,

Thanks for the heads up.

In the last email I suggested skip the leaf mappings in the split range 
in order to reduce page table walk overhead for split_mapping(start, 
end). In this way we can achieve:
    - reuse the most split code for repainting (just need 
NO_BLOCK_MAPPINGS | NO_CONT_MAPPINGS flag for repainting to split page 
table to PTEs)
    - just walk page table once
    - have similar page table walk overhead with 
split_mapping(start)/split_mapping(end) if the split range is large

I'm basically done on a new spin to implement it and solve all the 
review comments from v4. I should be able to post the new spin by the 
end of this week.

Regards,
Yang

> >