Re: [PATCH v4 0/7] use per-vma locks for /proc/pid/maps reads and PROCMAP_QUERY

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