Re: [PATCH] mm,hwpoison: return -EBUSY when page already poisoned

[Aili Yao]

On Tue, 2 Mar 2021 19:39:53 -0800
"Luck, Tony" <tony.luck@xxxxxxxxx> wrote:

> On Fri, Feb 26, 2021 at 10:59:15AM +0800, Aili Yao wrote:
> > Hi naoya, tony:  
> > > > 
> > > > Idea for what we should do next ... Now that x86 is calling memory_failure()
> > > > from user context ... maybe parallel calls for the same page should
> > > > be blocked until the first caller completes so we can:
> > > > a) know that pages are unmapped (if that happens)
> > > > b) all get the same success/fail status    
> > > 
> > > One memory_failure() call changes the target page's status and
> > > affects all mappings to all affected processes, so I think that
> > > (ideally) we don't have to block other threads (letting them
> > > early return seems fine).  Sometimes memory_failure() fails,
> > > but even in such case, PG_hwpoison is set on the page and other
> > > threads properly get SIGBUSs with this patch, so I think that
> > > we can avoid the worst scenario (like system stall by MCE loop).
> > >   
> > I agree with naoya's point, if we block for this issue, Does this change the result
> > that the process should be killed? Or is there something other still need to be considered?  
> 
> Ok ... no blocking ... I think someone in this thread suggested
> scanning the page tables to make sure the poisoned page had been
> unmapped.
> 
> There's a walk_page_range() function that does all the work for that.
> Just need to supply some callback routines that check whether a
> mapping includes the bad PFN and clear the PRESENT bit.
> 
> RFC patch below against v5.12-rc1
> 
> -Tony
> 
> From 8de23b7f1be00ad38e129690dfe0b1558fad5ff8 Mon Sep 17 00:00:00 2001
> From: Tony Luck <tony.luck@xxxxxxxxx>
> Date: Tue, 2 Mar 2021 15:06:33 -0800
> Subject: [PATCH] x86/mce: Handle races between machine checks
> 
> When multiple CPUs hit the same poison memory there is a race. The
> first CPU into memory_failure() atomically marks the page as poison
> and continues processing to hunt down all the tasks that map this page
> so that the virtual addresses can be marked not-present and SIGBUS
> sent to the task that did the access.
> 
> Later CPUs get an early return from memory_failure() and may return
> to user mode and access the poison again.
> 
> Add a new argument to memory_failure() so that it can indicate when
> the race has been lost. Fix kill_me_maybe() to scan page tables in
> this case to unmap pages.

> +
>  static void kill_me_now(struct callback_head *ch)
>  {
>  	force_sig(SIGBUS);
> @@ -1257,15 +1304,19 @@ static void kill_me_maybe(struct callback_head *cb)
>  {
>  	struct task_struct *p = container_of(cb, struct task_struct, mce_kill_me);
>  	int flags = MF_ACTION_REQUIRED;
> +	int already = 0;
>  
>  	pr_err("Uncorrected hardware memory error in user-access at %llx", p->mce_addr);
>  
>  	if (!p->mce_ripv)
>  		flags |= MF_MUST_KILL;
>  
> -	if (!memory_failure(p->mce_addr >> PAGE_SHIFT, flags) &&
> +	if (!memory_failure(p->mce_addr >> PAGE_SHIFT, flags, &already) &&
>  	    !(p->mce_kflags & MCE_IN_KERNEL_COPYIN)) {
> -		set_mce_nospec(p->mce_addr >> PAGE_SHIFT, p->mce_whole_page);
> +		if (already)
> +			walk_page_range(current->mm, 0, TASK_SIZE_MAX, &walk, (void *)(p->mce_addr >> PAGE_SHIFT));
> +		else
> +			set_mce_nospec(p->mce_addr >> PAGE_SHIFT, p->mce_whole_page);
>  		sync_core();
>  		return;

>  	MF_MUST_KILL = 1 << 2,
>  	MF_SOFT_OFFLINE = 1 << 3,
>  };

I have one doubt here, when "walk_page_range(current->mm, 0, TASK_SIZE_MAX, &walk, (void *)(p->mce_addr >> PAGE_SHIFT));" is done,
so how is the process triggering this error returned if it have taken the wrong data?

-- 
Thanks!
Aili Yao