[LU-13182] MAP_POPULATE hangs with Linux 5.4

Related — Wed, 29 Sep 2021 18:35:39 +0000

While testing Lustre with Linux 5.4 (kernel installed on top of Ubuntu 18.04), I came across an issue with mmap_cat test utility.

In this program, the call to mmap is:

mmappedData = mmap(NULL, filesize, PROT_READ, MAP_PRIVATE | MAP_POPULATE, fd, 0);

When using mmap_cat on a file, it hangs. However, if the call to mmap is changed as following, it works:

mmappedData = mmap(NULL, filesize, PROT_READ, MAP_PRIVATE, fd, 0);

So it seems the bug is related to the MAP_POPULATE flag, which semantic is:

MAP_POPULATE (since Linux 2.5.46)
              Populate (prefault) page tables for a mapping.  For a file
              mapping, this causes read-ahead on the file.  This will help
              to reduce blocking on page faults later.  MAP_POPULATE is sup‐
              ported for private mappings only since Linux 2.6.23.

Whamcloud Community JIRA

[LU-13182] MAP_POPULATE hangs with Linux 5.4