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

[Matthew Wilcox]

On Thu, Nov 09, 2023 at 09:47:24PM +0800, zhangpeng (AS) wrote:
> 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.

[...]

> 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?

Yes, this looks like a bug.

It seems to me that the easiest way to fix this is not to zero the pte
but to make it protnone?  That would send task 2 into do_numa_page()
where it would take the ptl, then check pte_same(), see that it's
changed and goto out, which will end up retrying the fault.

I'm not particularly expert at page table manipulation, so I'll let
somebody who is propose an actual patch.  Or you could try to do it?