Re: [mm] 408579cd62: WARNING:suspicious_RCU_usage

[Oliver Sang]

hi, Linus,

On Mon, Jul 03, 2023 at 07:29:48PM -0700, Linus Torvalds wrote:
> On Mon, 3 Jul 2023 at 18:48, Oliver Sang <oliver.sang@xxxxxxxxx> wrote:
> >
> > by patch [1], we found the warning is not fixed.
> 
> Hmm. I already committed that "fix" as obvious, since the main
> difference in commit 408579cd627a ("mm: Update do_vmi_align_munmap()
> return semantics") around that validate_mm() call was how it did that
> mmap_read_unlock().
> 
> > we also found there are some changes in stack backtrace. now it's as below:
> > (detail dmesg is attached)
> >
> > [   26.412372][    T1] stack backtrace:
> > [   26.412846][    T1] CPU: 0 PID: 1 Comm: systemd Not tainted 6.4.0-09908-gcb226fb1fb7a #1
> > [   26.413506][    T1] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.16.2-debian-1.16.2-1 04/01/2014
> > [   26.414326][    T1] Call Trace:
> > [   26.414605][    T1]  <TASK>
> > [   26.414847][    T1]  dump_stack_lvl+0x73/0xc0
> > [   26.415225][    T1]  lockdep_rcu_suspicious+0x1b7/0x280
> > [   26.415669][    T1]  mas_start+0x280/0x400
> > [   26.416037][    T1]  mas_find+0x27a/0x400
> > [   26.416391][    T1]  validate_mm+0x8b/0x2c0
> > [   26.416757][    T1]  __se_sys_brk+0xa35/0xc00
> 
> Ok, that is indeed a very different stack trace.
> 
> So maybe the fix is a real fix, but the first complaint shut up
> lockdep, so this is the *second* and unrelated complaint.
> 
> And indeed: it turns out that do_vma_munmap() does this:
> 
>         ret = do_vmi_align_munmap(vmi, vma, mm, start, end, uf, unlock);
>         validate_mm(mm);
> 
> and so we have *another* validate_mm() that is now done outside the lock.
> 
> That one is actually pretty pointless. We've *just* validated the mm
> already inside do_vmi_align_munmap(), except we only did it in one of
> the two return cases.
> 
> So I think the fix is to just move that validate_mm() into the other
> return case of do_vmi_align_munmap(), and remove it from the caller.
> 
> IOW, something like the attached (NOTE! This is in *addition* to the
> previous patch, which is the same as the one you quoted, just with
> slightly different whitespace as commit ae80b4041984: "mm: validate
> the mm before dropping the mmap lock").

Thanks a lot for guidance!
I applied below patch directly upon ae80b4041984, and confirmed the
WARNING gone. Thanks

> 
>                Linus

>  mm/mmap.c | 6 ++----
>  1 file changed, 2 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/mmap.c b/mm/mmap.c
> index 547b40531791..204ddcd52625 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -2571,6 +2571,7 @@ do_vmi_align_munmap(struct vma_iterator *vmi, struct vm_area_struct *vma,
>  	__mt_destroy(&mt_detach);
>  start_split_failed:
>  map_count_exceeded:
> +	validate_mm(mm);
>  	return error;
>  }
>  
> @@ -3019,12 +3020,9 @@ int do_vma_munmap(struct vma_iterator *vmi, struct vm_area_struct *vma,
>  		bool unlock)
>  {
>  	struct mm_struct *mm = vma->vm_mm;
> -	int ret;
>  
>  	arch_unmap(mm, start, end);
> -	ret = do_vmi_align_munmap(vmi, vma, mm, start, end, uf, unlock);
> -	validate_mm(mm);
> -	return ret;
> +	return do_vmi_align_munmap(vmi, vma, mm, start, end, uf, unlock);
>  }
>  
>  /*