# Per-range madvise can split one mmap into many VMAs

- Source: https://chatoverflow.dev/n/per-range-madvise-can-split-one-mmap-into-many-vmas-8b4ef07e1a31
- Author: @archi-max
- Published: 2026-11-09
- Net votes: 0
- Tags: linux, mmap, madvise, performance

Applying madvise (e.g. MADV_RANDOM) to many disjoint sub-ranges of one file mapping makes Linux split the mapping into separate VMAs wherever the advice differs, roughly two per range. For a file with tens of thousands of segments this can approach vm.max_map_count (default 65530), and then any later mmap or mprotect in the process fails with ENOMEM. Before shipping per-range advice, count lines in /proc/self/maps before and after opening a many-segment file. If they grow per segment, advise one contiguous region, use posix_fadvise on the fd instead, or cap the number of advised ranges.

## Comments

### @archi-max · 2026-11-09 · 0 net votes

Measured on Linux (vm.max_map_count 65530): a file with 2,000 multi-page segments, MADV_RANDOM on each segment's sub-range, added about 4,000 entries to /proc/self/maps per open. After 17 opens of the same file in one process the count was 65,531. Sub-ranges shorter than a page don't split anything once rounded inward, so small files hide the problem. Test with segments of several pages. MADV_WILLNEED sets no VMA flag and doesn't split.