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Date:   Fri, 2 Apr 2021 09:18:20 +0800
From:   Aili Yao <yaoaili@kingsoft.com>
To:     "Luck, Tony" <tony.luck@intel.com>
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Subject: Re: [PATCH v3] mm,hwpoison: return -EHWPOISON when page already
 poisoned
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On Thu, 1 Apr 2021 08:33:20 -0700
"Luck, Tony" <tony.luck@intel.com> wrote:

> On Wed, Mar 31, 2021 at 07:25:40PM +0800, Aili Yao wrote:
> > When the page is already poisoned, another memory_failure() call in the
> > same page now return 0, meaning OK. For nested memory mce handling, this
> > behavior may lead to one mce looping, Example:
> > 
> > 1.When LCME is enabled, and there are two processes A && B running on
> > different core X && Y separately, which will access one same page, then
> > the page corrupted when process A access it, a MCE will be rasied to
> > core X and the error process is just underway.
> > 
> > 2.Then B access the page and trigger another MCE to core Y, it will also
> > do error process, it will see TestSetPageHWPoison be true, and 0 is
> > returned.
> > 
> > 3.The kill_me_maybe will check the return:
> > 
> > 1244 static void kill_me_maybe(struct callback_head *cb)
> > 1245 {
> > 
> > 1254         if (!memory_failure(p->mce_addr >> PAGE_SHIFT, flags) &&
> > 1255             !(p->mce_kflags & MCE_IN_KERNEL_COPYIN)) {
> > 1256                 set_mce_nospec(p->mce_addr >> PAGE_SHIFT,
> > p->mce_whole_page);
> > 1257                 sync_core();
> > 1258                 return;
> > 1259         }
> > 
> > 1267 }
> 
> With your change memory_failure() will return -EHWPOISON for the
> second task that consumes poison ... so that "if" statement won't
> be true and so we fall into the following code:
> 
> 1273         if (p->mce_vaddr != (void __user *)-1l) {
> 1274                 force_sig_mceerr(BUS_MCEERR_AR, p->mce_vaddr, PAGE_SHIFT);
> 1275         } else {
> 1276                 pr_err("Memory error not recovered");
> 1277                 kill_me_now(cb);
> 1278         }
> 
> If this was a copy_from_user() machine check, p->mce_vaddr is set and
> the task gets a BUS_MCEERR_AR SIGBUS, otherwise we print that
> 
> 	"Memory error not recovered"
> 
> message and send a generic SIGBUS.  I don't think either of those options
> is right.
> 
> Combined with my "mutex" patch (to get rid of races where 2nd process returns
> early, but first process is still looking for mappings to unmap and tasks
> to signal) this patch moves forward a bit. But I think it needs an
> additional change here in kill_me_maybe() to just "return" if there is a
> EHWPOISON return from memory_failure()
> 
Got this, Thanks for your reply!
I will dig into this!

-- 
Thanks!
Aili Yao