Re: [PATCH 1/1] mm: do not increment pgfault stats when page fault handler retries

[Peter Xu]

Hi, Suren,

On Fri, Apr 14, 2023 at 10:54:44AM -0700, Suren Baghdasaryan wrote:
> If the page fault handler requests a retry, we will count the fault
> multiple times.  This is a relatively harmless problem as the retry paths
> are not often requested, and the only user-visible problem is that the
> fault counter will be slightly higher than it should be.  Nevertheless,
> userspace only took one fault, and should not see the fact that the
> kernel had to retry the fault multiple times.
> 
> Fixes: 6b4c9f446981 ("filemap: drop the mmap_sem for all blocking operations")
> Signed-off-by: Suren Baghdasaryan <surenb@xxxxxxxxxx>
> Reviewed-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
> ---
> Patch applies cleanly over linux-next and mm-unstable
> 
>  mm/memory.c | 16 ++++++++++------
>  1 file changed, 10 insertions(+), 6 deletions(-)
> 
> diff --git a/mm/memory.c b/mm/memory.c
> index 1c5b231fe6e3..d88f370eacd1 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -5212,17 +5212,16 @@ vm_fault_t handle_mm_fault(struct vm_area_struct *vma, unsigned long address,
>  
>  	__set_current_state(TASK_RUNNING);
>  
> -	count_vm_event(PGFAULT);
> -	count_memcg_event_mm(vma->vm_mm, PGFAULT);
> -
>  	ret = sanitize_fault_flags(vma, &flags);
>  	if (ret)
> -		return ret;
> +		goto out;
>  
>  	if (!arch_vma_access_permitted(vma, flags & FAULT_FLAG_WRITE,
>  					    flags & FAULT_FLAG_INSTRUCTION,
> -					    flags & FAULT_FLAG_REMOTE))
> -		return VM_FAULT_SIGSEGV;
> +					    flags & FAULT_FLAG_REMOTE)) {
> +		ret = VM_FAULT_SIGSEGV;
> +		goto out;
> +	}
>  
>  	/*
>  	 * Enable the memcg OOM handling for faults triggered in user
> @@ -5253,6 +5252,11 @@ vm_fault_t handle_mm_fault(struct vm_area_struct *vma, unsigned long address,
>  	}
>  
>  	mm_account_fault(regs, address, flags, ret);

Here is the mm_account_fault() function taking care of some other
accountings.  Perhaps good to put things into it?

It also already ignores invalid faults:

	if (ret & (VM_FAULT_ERROR | VM_FAULT_RETRY))
		return;

I see that you may also want to account for sigbus, however I really don't
know why.  Explanations would be great when it would matter.  So far it
makes sense to me if we skip both RETRY or ERROR cases.

> +out:
> +	if (!(ret & VM_FAULT_RETRY)) {
> +		count_vm_event(PGFAULT);
> +		count_memcg_event_mm(vma->vm_mm, PGFAULT);

There is one thing worth noticing is here vma may or may not be valid
depending on the retval of the fault.

RETRY is exactly one of the cases that accessing vma may be unsafe due to
releasing of mmap read lock.  The other one is the recently added
VM_FAULT_COMPLETE.  So if we want to move this chunk (or any vma reference)
to be later we need to consider a valid vma / mm being there first, or
we're prone to accessing a vma that has already been released, I think.

> +	}
>  
>  	return ret;
>  }
> -- 
> 2.40.0.634.g4ca3ef3211-goog
> 
> 

Thanks,

-- 
Peter Xu