Hi Suren,
I promised I'd share VMA merging scenarios so we can be absolutely sure we have
all cases covered, I share that below. I also included information on split.
Hopefully this is useful! And maybe we can somehow put in a comment or commit
msg or something somewhere? Not sure if a bit much for that though :)
Note that in all of the below we hold exclusive mmap, vma + rmap write locks.
## Merge with change to EXISTING VMA
### Merge both
start end
|<---->|
|-------********-------|
prev middle next
extend delete delete
1. Set prev VMA range [prev->vm_start, next->vmend)
2. Overwrite prev, middle, next nodes in maple tree with prev
3. Detach middle VMA
4. Free middle VMA
5. Detach next VMA
6. Free next VMA
### Merge left full
start end
|<--------->|
|-------*************
prev middle
extend delete
1. Set prev VMA range [prev->vm_start, end)
2. Overwrite prev, middle nodes in maple tree with prev
3. Detach middle VMA
4. Free middle VMA
### Merge left partial
start end
|<---->|
|-------*************
prev middle
extend partial overwrite
1. Set prev VMA range [prev->vm_start, end)
2. Set middle range [end, middle->vm_end)
3. Overwrite prev, middle (partial) nodes in maple tree with prev
### Merge right full
start end
|<--------->|
*************-------|
middle next
delete extend
1. Set next range [start, next->vm_end)
2. Overwrite middle, next nodes in maple tree with next
3. Detach middle VMA
4. Free middle VMA
### Merge right partial
start end
|<----->|
*************-------|
middle next
shrink extend
1. Set middle range [middle->vm_start, start)
2. Set next range [start, next->vm_end)
3. Overwrite middle (partial), next nodes in maple tree with next
## Merge due to introduction of proposed NEW VMA
These cases are easier as there's no existing VMA to either remove or partially
adjust.
### Merge both
start end
|<------>|
|-------..........-------|
prev (proposed) next
extend delete
1. Set prev VMA range [prev->vm_start, next->vm_end)
2. Overwrite prev, next nodes in maple tree with prev
3. Detach next VMA
4. Delete next VMA
### Merge left
start end
|<------>|
|-------..........
prev (proposed)
extend
1. Set prev VMA range [prev->vm_start, end)
2. Overwrite prev node in maple tree with newly extended prev
(This is what's used for brk() and bprm_mm_init() stack relocation in
relocate_vma_down() too)
### Merge right
start end
|<------>|
..........-------|
(proposed) next
extend
1. Set next VMA range [start, next->vm_end)
2. Overwrite next node in maple tree with newly extended next
## Split VMA
If new below:
addr
|-----.-----|
| new . |
|-----.-----|
vma
Otherwise:
addr
|-----.-----|
| . new |
|-----.-----|
vma
1. Duplicate vma
2. If new below, set new range to [vma-vm_start, addr)
3. Otherwise, set new range to [addr, vma->vm_end)
4. If new below, Set vma range to [addr, vma->vm_end)
5. Otherwise, set vma range to [vma->vm_start, addr)
6. Partially overwrite vma node in maple tree with new
Cheers, Lorenzo
