[Question]: major faults are still triggered after mlockall when numa balancing

[zhangpeng (AS)]

Hi everyone,

There is a performance issue that has been bothering us recently.
This problem can reproduce in the latest mainline version (Linux 6.6).

We use mlockall(MCL_CURRENT | MCL_FUTURE) in the user mode process
to avoid performance problems caused by major fault.

There is a stage in numa fault which will set pte as 0 in do_numa_page() :
ptep_modify_prot_start() will clear the vmf->pte, until
ptep_modify_prot_commit() assign a value to the vmf->pte.

For the data segment of the user-mode program, the global variable area
is a private mapping. After the pagecache is loaded, the private
anonymous page is generated after the COW is triggered. Mlockall can
lock COW pages (anonymous pages), but the original file pages cannot
be locked and may be reclaimed. If the global variable (private anon page)
is accessed when vmf->pte is zero which is concurrently set by numa fault,
a file page fault will be triggered.

At this time, the original private file page may have been reclaimed.
If the page cache is not available at this time, a major fault will be
triggered and the file will be read, causing additional overhead.

Our problem scenario is as follows:

task 1                      task 2
------                      ------
/* scan global variables */
do_numa_page()
  spin_lock(vmf->ptl)
  ptep_modify_prot_start()
  /* set vmf->pte as null */
                            /* Access global variables */
                            handle_pte_fault()
                              /* no pte lock */
                              do_pte_missing()
                                do_fault()
                                  do_read_fault()
  ptep_modify_prot_commit()
  /* ptep update done */
  pte_unmap_unlock(vmf->pte, vmf->ptl)
                                    do_fault_around()
                                    __do_fault()
                                      filemap_fault()
                                        /* page cache is not available
                                        and a major fault is triggered */
                                        do_sync_mmap_readahead()
                                        /* page_not_uptodate and goto
                                        out_retry. */

Is there any way to avoid such a major fault?

--
Best Regards,
Peng