[GitHub] [nuttx] CV-Bowen opened a new pull request, #8353: Reduce the memory node overhead size

[via GitHub]

CV-Bowen opened a new pull request, #8353:
URL: https://github.com/apache/nuttx/pull/8353

   ## Summary
   This PR reduces the memory node overhead size to half of the original by 
move the `node->preceding` to previous free node in physical like TLSF does 
(http://www.gii.upv.es/tlsf/main/docs).
   
   Original memory layout of Allocnode and Freenode:
   
![image](https://user-images.githubusercontent.com/39286361/215438882-1825529a-3fdf-4fc0-8cc2-58dfd0e99234.png)
   
   This PR contains two commits:
   Commit 1: Exchange `node->size` and `node->preceding` position and move the 
MM_ALLOC_BIT to `node->size`, this commit is a preparation for move the 
`node->preceding` to previous free node in physical address. 
   After Commit 1, the memory layout of allocnode and freenode become:
   
![image](https://user-images.githubusercontent.com/39286361/215438963-35176f4b-5726-47d7-8f86-f36eeb924052.png)
   
   Commit 2: Add MM_PREVFREE_BIT in `node->size` and move the `node->preceding` 
to previous free node in physical. 
   After Commit 2, the memory layout of allocnode and freenode become:
   
![image](https://user-images.githubusercontent.com/39286361/215439734-f366b1f9-7166-4249-9384-c5c563c50205.png)
   
   ## Impact
   Dynamic Memory Managerment
   
   ## Testing
   Vela and sim mmtest
   


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