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

[Yang Shi]

On Thu, Nov 9, 2023 at 5:57 PM Yin, Fengwei <fengwei.yin@xxxxxxxxx> wrote:
>
>
>
> On 11/10/2023 6:54 AM, Yang Shi wrote:
> > On Thu, Nov 9, 2023 at 5:48 AM zhangpeng (AS) <zhangpeng362@xxxxxxxxxx> wrote:
> >>
> >> 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?
> >
> > IMHO I don't think it is a bug. The man page quoted by Willy says "All
> > mapped pages are guaranteed to be resident in RAM when the call
> > returns successfully", but the later COW already made the file page
> > unmapped, right? The PTE pointed to the COW'ed anon page.
> > Hypothetically if we kept the file page mlocked and unmapped,
> > munlock() would have not munlocked the file page at all, it would be
> > mlocked in memory forever.
> But in this case, even the COW page is mlocked. There is small window
> that PTE is set to null in do_numa_page(). data segment access (it's to
> COW page which has nothing to do with original page cache) happens in
> this small window will trigger filemap_fault() to fault in original
> page cache.

Yes, my point is this may not break the mlockall, but the potential
optimization by avoiding the major fault may still stand.

>
> I had thought to do double check whether vmf->pte is NULL in do_read_fault().
> But it's not reliable enough.
>
> Matthew's idea to use protnone to block both hardware accessing and
> do_pte_missing() looks more promising to me.
>
>
> Regards
> Yin, Fengwei
>
> >
> >>
> >> --
> >> Best Regards,
> >> Peng
> >>