Re: subobj-rmap

["Martin J. Bligh" <mbligh@aracnet.com>]

>> 0-150 -> 150-200 -> 200-300 -> 300-400 -> 400-500 -> 500-999
>>  A          A          A          A          A          A
>>  B          B
>>             C          C          C 
>>                                   D          D          
>>                                   E          E          
>>  F          F          F          F          F          F
> 
> I thought of that but decided it is too simple :)
> 
> A downside with it is that from time to time you need to split or
> merge subobjects, and that means splitting or merging the list nodes
> linking "rows" in the table above - potentially quite a lot of memory
> allocation and traversal for a single mmap().

The amount of work to be done is still fairly small ... and we already
do (as far as I can see) *exactly* this already for the existing
rb tree. Yes, mmap has a little bit more overhead, but you lose all
the per-page stuff, which seems much more efficient to me.
 
>> We can always leave the sys_remap_file_pages stuff using pte_chains,
>> and should certainly do that at first. But doing it for normal stuff
>> should be less controversial, I think.
> 
> If you implement the 2d data structure that you illustrated, you have
> a list node for each point in the table.
> 
> By the time your subobject regions are 1 page wide, you have a data
> structure that is order-equivalent to pte rmap chains, although the
> exact number of words is likely to be higher.

Well, yes. Except I hope nobody would want to do that on a per-page
basis. If you want that level of granularity, we should just do this 
for linear objects, and fall back to pte_chains for nonlinear.

Life would be a whole lot simpler if people were willing to specify
non-linear VMAs at create time - I don't see that as a big burden,
personally. That'd get rid of all the conversion stuff.

M.

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