qbittorrent: file_mmap (/path/to/file) error: Out of memory”

To solve qbittorrent error "file_mmap error: Out of memory" in Linux with libtorrent 2.0.x:

Option - Advanced - File pool size -> reduce to a smaller number, for example 40. In my case, 20 does not work, but 10 works. Also check https://github.com/arvidn/libtorrent/issues/6561#issuecomment-1014669417 for an detailed explanation of the option.

more aggressive approach: Option - Advanced - Disk IO Type -> change to "POSIX compliant" (haven't tried myself, but in theory this should disable file_mmap and hence prevent the error)