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

[HORIGUCHI NAOYA(堀口 直也)]

On Thu, Feb 25, 2021 at 10:15:42AM -0800, Luck, Tony wrote:
> On Thu, Feb 25, 2021 at 12:38:06PM +0000, HORIGUCHI NAOYA(堀口 直也) wrote:
> > Thank you for shedding light on this, this race looks worrisome to me.
> > We call try_to_unmap() inside memory_failure(), where we find affected
> > ptes by page_vma_mapped_walk() and convert into hwpoison entires in
> > try_to_unmap_one().  So there seems two racy cases:
> > 
> >   1)
> >      CPU 0                          CPU 1
> >      page_vma_mapped_walk
> >                                     clear _PAGE_PRESENT bit
> >        // skipped the entry
> > 
> >   2)
> >      CPU 0                          CPU 1
> >      page_vma_mapped_walk
> >        try_to_unmap_one
> >                                     clear _PAGE_PRESENT bit
> >          convert the entry
> >          set_pte_at
> > 
> > In case 1, the affected processes get signals on later access,
> > so although the info in SIGBUS could be different, that's OK.
> > And we have no impact in case 2.
> 
> I've been debugging a similar issue for a few days and finally
> got enough traces to partially understand what happened.
> 
> The test case is a multi-threaded pointer chasing micro-benchmark
> running on all logical CPUs. We then inject poison into the address
> space of the process.
> 
> All works fine if one thread consumes poison and completes all
> Linux machine check processing before any other threads read the
> poison. The page is unmapped, a SIGBUS is sent (which kills all
> threads).
> 
> But in the problem case I see:

Thanks for the description, it's helpful to understand the problem.

> 
> CPU1 reads poison, takes a machine check. Gets to the
> kill_me_maybe() task work, which calls memory_failure()
> this CPU sets the page poison flag, but is still executing the
> rest of the flow to hunt down tasks/mappings to invalidate pages
> and send SIGBUS if required.
> 
> CPU2 reads the poison. When it gets to memory_failure()
> there's an early return because the poison flag is already
> set. So in current code it returns and takes the machine
> check again.
> 
> CPU3 reads the poison and starts along same path that CPU2
> did.

I think that the MCE loop happening on CPU2 and CPU3 is unexpected
and these threads should immediately kill the current process on
each CPU.  force_sig_mceerr() in kill_me_maybe() is supposed to do it,
so Aili's patch would fix this issue too?

> 
> Meanwhile CPU1 gets far enough along in memory failure and hits
> a problem. It prints:
> 
> [ 1867.409837] Memory failure: 0x42a9ff6: reserved kernel page still referenced by 1 users
> [ 1867.409850] Memory failure: 0x42a9ff6: recovery action for reserved kernel page: Failed
> 
> and doesn't complete unmapping the page that CPU2 and CPU3 are touching.
> 
> Other CPUs gradually reach the poison and join in the fun of repeatedly
> taking machine checks.
> 
> I have not yet tracked why this user access is reporting as a "reserved kernel page".
> Some traces showed that futex(2) syscall was in use from this benchmark,
> so maybe the kernel locked a user page that was a contended futex???

This might imply that current logic to identify page state does
not work properly on this exotic type of user page, I'll take a
look on this from futex's viewpoint.

> 
> 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).

Thanks,
Naoya Horiguchi