Re: [PATCH v5] mm/gup: check page hwposion status for coredump.

[David Hildenbrand]

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@xxxxxxx> 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@xxxxxxxxxx>).
> > > > > >
> > > > > > Link: https://lkml.kernel.org/r/20210319104437.6f30e80d@alex-virtual-machine
> > > > > > Signed-off-by: Aili Yao <yaoaili@xxxxxxxxxxxx>
> > > > > > Cc: David Hildenbrand <david@xxxxxxxxxx>
> > > > > > Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
> > > > > > Cc: Naoya Horiguchi <naoya.horiguchi@xxxxxxx>
> > > > > > Cc: Oscar Salvador <osalvador@xxxxxxx>
> > > > > > Cc: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
> > > > > > Cc: Aili Yao <yaoaili@xxxxxxxxxxxx>
> > > > > > Cc: stable@xxxxxxxxxxxxxxx
> > > > > > Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> > > > > > ---
> > > > > >     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.

--
Thanks,

David / dhildenb