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    SubjectRe: [PATCH v2] mm,hwpoison: return -EBUSY when page already poisoned
    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@kingsoft.com>
    >
    > Reviewed-by: Naoya Horiguchi <naoya.horiguchi@nec.com>

    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].

    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.
    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).

    #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.

    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?

    -Tony

    [1] still looking at why my futex injection test ends with a "reserved
    kernel page still referenced by 1 users"

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