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

[Matthew Wilcox <willy@infradead.org>]

I went spelunking to try to find out more about this issue, and I
discovered it's Aneesh's fault from 2017 ...

On Thu, Nov 09, 2023 at 09:47:24PM +0800, zhangpeng (AS) 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?
> 
> -- 
> Best Regards,
> Peng
>