Verifying HugePages Behavior in XFS Files

For best performance, map the data files placed in DAX-enabled storage in to the virtual address space using HugePages mappings. For a memory-mapped file, set up the virtual-to-physical address translations when the pages are first touched.

If you map pages through DAX using HugePages mappings, then the pages will not use page-cache pages.

You can enable a trace to determine if the page-fault handler granularity is 2 MB or lower, and if it bypasses the page cache.

1. To view this trace, log in as root and run the following trace enabling command:

   # echo 1 >
   /sys/kernel/debug/tracing/events/fs_dax/dax_pmd_fault_done/
   enable

2. Log out from root and create a file in the DAX-mounted XFS file system and run mkfs on it using the omsfscmds utility.
3. Log in again as root and run the following command to look at the generated trace:

   # cat /sys/kernel/debug/tracing/trace

   Regular 2 MB page faults that bypass the page cache will show the following trace with NOPAGE at the end of the trace:

   oms_test-21456 [024] .... 13566209.645007: dax_pmd_fault_done:
   dev 259:10 ino 0x2005 shared ALLOW_RETRY|KILLABLE|USER address
   0x7fb717e01000 vm_start 0x7fb717e00000 vm_end 0x7fb997e00000
   pgoff 0x1 max_pgoff 0x280000 NOPAGE

   If the page fault occurs at a lower granularity of 4 KB, then you will see a FALLBACK in the page fault handler trace. This indicates that 2 MB granularity page fault handlers failed and the fallback path through the page cache is executed. Systems showing a FALLBACK trace must not be used to run OMS.

   oms_test-21456 [024] .... 13566209.646801: dax_pmd_fault_done:
   dev 259:9 ino 0xc3 shared WRITE|ALLOW_RETRY|KILLABLE|USER
   address 0x7fb717e0c000 vm_start 0x7fb717e00000 vm_end
   0x7fb997e00000 pgoff 0xc max_pgoff 0x280000 FALLBACK
   

4. Tracing substantially impinges on performance. You must disable tracing before running workloads. To disable the trace, log in as root and run the following command:

   # echo 0 >
   /sys/kernel/debug/tracing/events/fs_dax/dax_pmd_fault_done/
   enable