Re: [PATCH v2] mm,hwpoison: return -EBUSY when page already poisoned

[HORIGUCHI NAOYA(堀口 直也)]

On Tue, Mar 09, 2021 at 12:01:40PM -0800, Luck, Tony wrote:
> On Tue, Mar 09, 2021 at 08:28:24AM +0000, HORIGUCHI NAOYA(堀口 直也) wrote:
> > On Tue, Mar 09, 2021 at 02:35:34PM +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 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 }
> > > 
> > > 4. The error process for B will end, and may nothing happened if
> > > kill-early is not set, The process B will re-excute instruction and get
> > > into mce again and then loop happens. And also the set_mce_nospec()
> > > here is not proper, may refer to commit fd0e786d9d09 ("x86/mm,
> > > mm/hwpoison: Don't unconditionally unmap kernel 1:1 pages").
> > > 
> > > For other cases which care the return value of memory_failure() should
> > > check why they want to process a memory error which have already been
> > > processed. This behavior seems reasonable.
> > 
> > Other reviewers shared ideas about the returned value, but actually
> > I'm not sure which the best one is (EBUSY is not that bad).
> > What we need to fix the reported issue is to return non-zero value
> > for "already poisoned" case (the value itself is not so important). 
> > 
> > Other callers of memory_failure() (mostly test programs) could see
> > the change of return value, but they can already see EBUSY now and
> > anyway they should check dmesg for more detail about why failed,
> > so the impact of the change is not so big.
> > 
> > > 
> > > Signed-off-by: Aili Yao <yaoaili@xxxxxxxxxxxx>
> > 
> > Reviewed-by: Naoya Horiguchi <naoya.horiguchi@xxxxxxx>
> 
> I think that both this and my "add a mutex" patch are both
> too simplistic for this complex problem :-(
> 
> When multiple CPUs race to call memory_failure() for the same
> page we need the following results:
> 
> 1) Poison page should be marked not-present in all tasks
> 	I think the mutex patch achieves this as long as
> 	memory_failure() doesn't hit an error[1].

My assumption is that reserved kernel pages is not supposed to be mapped to any
process, so once memory_failure() judges a page as such, we never mark any page
table entry to hwpoison entry, is that correct?  So my question is why some
user-mapped page was judged as "reserved kernel page".  Futex allows such a situation?

I personally tried some testcase crossing futex and hwpoison, but I can't
reproduced "reserved kernel page" case.  If possible, could you provide me
with a little more detail about your testcase?

> 
> 2) All tasks that were executing an instruction that was accessing
>    the poison location should see a SIGBUS with virtual address and
>    BUS_MCEERR_AR signature in siginfo.
> 	Neither solution achieves this. The -EBUSY return ensures
> 	that there is a SIGBUS for the tasks that get the -EBUSY
> 	return, but no siginfo details.

Yes, that's not yet perfect but avoiding MCE loop is a progress.

> 	Just the mutex patch *might* have BUS_MCEERR_AO signature
> 	to the race losing tasks, but only if they have PF_MCE_EARLY
> 	set (so says the comment in kill_proc() ... but I don't
> 	see the code checking for that bit).

commit 30c9cf49270 might explain this, task_early_kill() got to call
find_early_kill_thread() (checking PF_MCE_EARLY) in this case.

> 
> #2 seems hard to achieve ... there are inherent races that mean the
> AO SIGBUS could have been queued to the task before it even hits
> the poison.

So I feel that we might want some change on memory_failure() to send
SIGBUS(BUS_MCEERR_AR) to "race losing tasks" within the new mutex.
I agree that how we find the error address it also a problem.
For now, I still have no better idea than page table walk.

> 
> Maybe should include a non-action:
> 
> 3) A task should only see one SIGBUS per poison?
> 	Not sure if this is achievable either ... what if the task
> 	has the same page mapped multiple times?

My thought is that hwpoison-aware applications could have dedlicated thread
for SIGBUS handling, so it's better to be prepared for multiple signals for
the same error event.

Thanks,
Naoya Horiguchi