On Fri, Jun 13, 2025 at 8:01 AM Lorenzo Stoakes <lorenzo.stoa...@oracle.com> wrote: > > 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.
Thanks Lorenzo! This is great and very helpful. > > 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 :) I'll see if I can add a short version into my next cover letter. > > 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 This case should be fine with per-vma locks while reading /proc/pid/maps. In the worst case we will report some of the original vmas before the merge and then the final merged vma, so prev might be seen twice but no gaps should be observed. > > ### 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 Same as the previous case. Worst case we report prev twice - once before the merge, once after the merge. > > ### 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 We might report prev twice here and this might cause us to retry if we see a temporary gap between old prev and new middle vma. But retry should handle this case, so I think we are good here. > > ### 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 Worst case we report middle twice. > > ### 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 Worse case we retry and report middle twice. > > ## 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 Worst case we report prev twice after retry. > > ### 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 Worst case we report prev twice. > > (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 This will show either a legit gap + original next or the extended next with no gap. Both ways we are fine. > > ## 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 These are fine too. We will either report before-split view or after-split view. Thanks, Suren. > > Cheers, Lorenzo
