Date:   Wed, 31 Mar 2021 15:07:32 +0800
From:   Aili Yao <yaoaili@...gsoft.com>
To:     David Hildenbrand <david@...hat.com>,
        "HORIGUCHI NAOYA堀口 直也)" 
        <naoya.horiguchi@....com>
CC:     Matthew Wilcox <willy@...radead.org>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "yangfeng1@...gsoft.com" <yangfeng1@...gsoft.com>,
        "sunhao2@...gsoft.com" <sunhao2@...gsoft.com>,
        Oscar Salvador <osalvador@...e.de>,
        Mike Kravetz <mike.kravetz@...cle.com>, <yaoaili@...gsoft.com>
Subject: Re: [PATCH v5] mm/gup: check page hwposion status for coredump.

On Wed, 31 Mar 2021 08:44:53 +0200
David Hildenbrand <david@...hat.com> wrote:

> On 31.03.21 06:32, HORIGUCHI NAOYA(堀口 直也) wrote:
> > On Wed, Mar 31, 2021 at 10:43:36AM +0800, Aili Yao wrote:  
> >> On Wed, 31 Mar 2021 01:52:59 +0000 HORIGUCHI NAOYA(堀口　直也) <naoya.horiguchi@....com> wrote:  
> >>> On Fri, Mar 26, 2021 at 03:22:49PM +0100, David Hildenbrand wrote:  
> >>>> On 26.03.21 15:09, David Hildenbrand wrote:  
> >>>>> On 22.03.21 12:33, Aili Yao wrote:  
> >>>>>> When we do coredump for user process signal, this may be one SIGBUS signal
> >>>>>> with BUS_MCEERR_AR or BUS_MCEERR_AO code, which means this signal is
> >>>>>> resulted from ECC memory fail like SRAR or SRAO, we expect the memory
> >>>>>> recovery work is finished correctly, then the get_dump_page() will not
> >>>>>> return the error page as its process pte is set invalid by
> >>>>>> memory_failure().
> >>>>>>
> >>>>>> But memory_failure() may fail, and the process's related pte may not be
> >>>>>> correctly set invalid, for current code, we will return the poison page,
> >>>>>> get it dumped, and then lead to system panic as its in kernel code.
> >>>>>>
> >>>>>> So check the hwpoison status in get_dump_page(), and if TRUE, return NULL.
> >>>>>>
> >>>>>> There maybe other scenario that is also better to check hwposion status
> >>>>>> and not to panic, so make a wrapper for this check, Thanks to David's
> >>>>>> suggestion(<david@...hat.com>).
> >>>>>>
> >>>>>> Link: https://lkml.kernel.org/r/20210319104437.6f30e80d@alex-virtual-machine
> >>>>>> Signed-off-by: Aili Yao <yaoaili@...gsoft.com>
> >>>>>> Cc: David Hildenbrand <david@...hat.com>
> >>>>>> Cc: Matthew Wilcox <willy@...radead.org>
> >>>>>> Cc: Naoya Horiguchi <naoya.horiguchi@....com>
> >>>>>> Cc: Oscar Salvador <osalvador@...e.de>
> >>>>>> Cc: Mike Kravetz <mike.kravetz@...cle.com>
> >>>>>> Cc: Aili Yao <yaoaili@...gsoft.com>
> >>>>>> Cc: stable@...r.kernel.org
> >>>>>> Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
> >>>>>> ---
> >>>>>>     mm/gup.c      |  4 ++++
> >>>>>>     mm/internal.h | 20 ++++++++++++++++++++
> >>>>>>     2 files changed, 24 insertions(+)
> >>>>>>
> >>>>>> diff --git a/mm/gup.c b/mm/gup.c
> >>>>>> index e4c224c..6f7e1aa 100644
> >>>>>> --- a/mm/gup.c
> >>>>>> +++ b/mm/gup.c
> >>>>>> @@ -1536,6 +1536,10 @@ struct page *get_dump_page(unsigned long addr)
> >>>>>>     				      FOLL_FORCE | FOLL_DUMP | FOLL_GET);
> >>>>>>     	if (locked)
> >>>>>>     		mmap_read_unlock(mm);  
> >>>>>
> >>>>> Thinking again, wouldn't we get -EFAULT from __get_user_pages_locked()
> >>>>> when stumbling over a hwpoisoned page?
> >>>>>
> >>>>> See __get_user_pages_locked()->__get_user_pages()->faultin_page():
> >>>>>
> >>>>> handle_mm_fault()->vm_fault_to_errno(), which translates
> >>>>> VM_FAULT_HWPOISON to -EFAULT, unless FOLL_HWPOISON is set (-> -EHWPOISON)
> >>>>>
> >>>>> ?  
> >>>
> >>> We could get -EFAULT, but sometimes not (depends on how memory_failure() fails).
> >>>
> >>> If we failed to unmap, the page table is not converted to hwpoison entry,
> >>> so __get_user_pages_locked() get the hwpoisoned page.
> >>>
> >>> If we successfully unmapped but failed in truncate_error_page() for example,
> >>> the processes mapping the page would get -EFAULT as expected.  But even in
> >>> this case, other processes could reach the error page via page cache and
> >>> __get_user_pages_locked() for them could return the hwpoisoned page.
> >>>  
> >>>>
> >>>> Or doesn't that happen as you describe "But memory_failure() may fail, and
> >>>> the process's related pte may not be correctly set invalid" -- but why does
> >>>> that happen?  
> >>>
> >>> Simply because memory_failure() doesn't handle some page types like ksm page
> >>> and zero page. Or maybe shmem thp also belongs to this class.  
> 
> Thanks for that info!
> 
> >>>  
> >>>>
> >>>> On a similar thought, should get_user_pages() never return a page that has
> >>>> HWPoison set? E.g., check also for existing PTEs if the page is hwpoisoned?  
> >>>
> >>> Make sense to me. Maybe inserting hwpoison check into follow_page_pte() and
> >>> follow_huge_pmd() would work well.  
> >>
> >> I think we should take more care to broadcast the hwpoison check to other cases,
> >> SIGBUS coredump is such a case that it is supposed to not touch the poison page,
> >> and if we return NULL for this, the coredump process will get a successful finish.
> >>
> >> Other cases may also meet the requirements like coredump, but we need to identify it,
> >> that's the poison check wrapper's purpose. If not, we may break the integrity of the
> >> related action, which may be no better than panic.  
> > 
> > If you worry about regression and would like to make this new behavior conditional,
> > we could use FOLL_HWPOISON to specify that the caller is hwpoison-aware so that
> > any !FOLL_HWPOISON caller ignores the hwpoison check and works as it does now.
> > This approach looks to me helpful because it would encourage developers touching
> > gup code to pay attention to FOLL_HWPOISON code.  
> 
> FOLL_HWPOISON might be the right start, indeed.
> 

Got this, Thanks!
I will dig more!

-- 
Thanks!
Aili Yao

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