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Add diagram to doc

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Mathieu Maret 2022-07-30 15:59:50 +02:00
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docs/3_virtual_mem.md
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@ -27,12 +27,13 @@ A given entry N in the PD point to the PD (this is possible because PDE very loo
Then, accessing N * 4Mo + I * 4Ko is accessing the PT of the Ieme entry in the PD (as MMU take the PD pointed by the PDE number N like a PT).
More particularly, accessing N * 4Mo + N * 4ko is accessing the PD.
PD is at Vaddr N * 4Mo and take 4ko. Each PT are allocated dynamically.
Just make sure that N have not been used by identity mapping
PD is at Vaddr N * 4Mo and take 4ko. Each PT is allocated dynamically.
Just make sure that N have not been used by identity mapping.
## Virtual memory allocators
We will setup 2 different virtual memory allocator:
We will setup 2 different virtual memory allocators:
* `allocArea` for large memory area of several PAGE_SIZE size
* `alloc` for object of any size
@ -44,10 +45,11 @@ An area is represented by `struct memArea` and basically consist of a virtual me
This is a simple allocator keeping 2 linked list of used/free area.
Allocating a new area (thanks to `areaAlloc()`) consist of:
1. finding an area big enough in the free list.
2. split it if the found area is too large.
3. put the found area in the user area list.
4. optionally map the area to physical page
1. Finding an area big enough in the free list.
2. Split it if the found area is too large.
3. Put the found area in the user area list.
4. Optionally map the area to physical pages.
Freeing an area (thank to `areaFree()`) consist of trying to find adjacent free area and merge them with this one or just adding the area to the free one.
@ -57,9 +59,38 @@ Freeing an area (thank to `areaFree()`) consist of trying to find adjacent free
For large object allocation ( > PAGE_SIZE), this a basically a call to `areaAlloc()`.
For smaller object, this is a more complex allocator based on the concept of slab.
The allocator is configured at startup to maintain slab for allocating object of a given size.
So a slab is a linked list of `struct slabEntry` having each of the entries pointing to one or several memory area that is divided in chunk of the slab configured size.
It's a linked list so it is each to add a new `struct slabEntry` when one is full.
The allocator is configured at startup to maintain a collection of slabs, each of them configured for allocating object of a given size.
So a slab is described by `struct slabDesc` and contains a linked list of `struct slabEntry`.
Each of the `struct slabEntry` entry point to a memory area that is divided in chunk of the slab configured size.
It's a linked list so it is easy to add a new `struct slabEntry` when one is full.
Inside a `struct slabEntry` the `freeEl` attribute point to the next free chunk (of the slab configured size) in the allocated page(s). At this address is also a pointer to the next free area in this area.
.-----------------------------.
| struct slabEntry { |
| vaddr_t page; |-------------------->.---------------------.
| void *freeEl; |-------------. |.------------------. |
| size_t size; | | || allocated chunk | |
| bool_t full; | | |.-------------------.|
| struct slabEntry *next; || -------->| addr of next free |----.
| struct slabEntry *prev; || |.------------------.'| |
| }; || || allocated chunk | | |
'-----------------------------'| |.------------------. | |
.------------------------------' || NULL |<----'
'-->.-----------------------------. |.------------------. |
| struct slabEntry { | || allocated chunk | |
| vaddr_t page; | |'------------------' |
| void *freeEl; | '---------------------'
| size_t size; |
| bool_t full; |
| struct slabEntry *next; ||
| struct slabEntry *prev; ||
| }; ||
'-----------------------------'|
.----------------------------------'
'-->.-----.
| ... |
'-----'
So there is one slab for object of 4B, one for 126B ...
The linked-list containing all the slab (`struct slabDesc`) is called the slub.

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