[LU-13182] MAP_POPULATE hangs with Linux 5.4

• Type: Bug
• Resolution: Fixed
• Priority: Minor
• Fix Version/s: Lustre 2.14.0, Lustre 2.12.7
• Affects Version/s: Lustre 2.14.0
• Severity: 3
• Assignee: Oleg Drokin
• Reporter: Sebastien Buisson

Description

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.

Issue Links

• is related to: LU-14021 NULL pointer dereference in _raw_write_lock
• is related to: LU-14022 sanity-hsm test 1a hung at verifying released state
• is related to: LU-13740 Ubuntu 20.04 LTS release