That’s why you do your own memory accounting in the database. Of course, that assumes you own the machine; for an “embedded” DB like LMDB something like PSI may be necessary.
Another possibility for reclaiming physical memory beside unused page decommit with MADV_FREE/MADV_DONTNEED (there might not be unused pages to decommit) is to manually page out cold anonymous pages with MADV_COLD/MADV_PAGEOUT (thanks Android). You can combine this with low swappiness so anonymous pages are unlikely to be paged out automatically when there are clean file-backed pages that can be reclaimed.
Another possibility for reclaiming physical memory beside unused page decommit with MADV_FREE/MADV_DONTNEED (there might not be unused pages to decommit) is to manually page out cold anonymous pages with MADV_COLD/MADV_PAGEOUT (thanks Android). You can combine this with low swappiness so anonymous pages are unlikely to be paged out automatically when there are clean file-backed pages that can be reclaimed.