Re: [PATCHv5 2/2] powerpc/pseries: update device tree before ejecting hotplug uevents

[Pingfan Liu <kernelfans@gmail.com>]

On Sat, Apr 10, 2021 at 12:33 AM Michal Suchánek <msuchanek@suse.de> wrote:
>
> Hello,
>
> On Fri, Aug 28, 2020 at 04:10:09PM +0800, Pingfan Liu wrote:
> > On Thu, Aug 27, 2020 at 3:53 PM Laurent Dufour <ldufour@linux.ibm.com> wrote:
> > >
> > > Le 10/08/2020 à 10:52, Pingfan Liu a écrit :
> > > > A bug is observed on pseries by taking the following steps on rhel:
> > > > -1. drmgr -c mem -r -q 5
> > > > -2. echo c > /proc/sysrq-trigger
> > > >
> > > > And then, the failure looks like:
> > > > kdump: saving to /sysroot//var/crash/127.0.0.1-2020-01-16-02:06:14/
> > > > kdump: saving vmcore-dmesg.txt
> > > > kdump: saving vmcore-dmesg.txt complete
> > > > kdump: saving vmcore
> > > >   Checking for memory holes                         : [  0.0 %] /                   Checking for memory holes                         : [100.0 %] |                   Excluding unnecessary pages                       : [100.0 %] \                   Copying data                                      : [  0.3 %] -          eta: 38s[   44.337636] hash-mmu: mm: Hashing failure ! EA=0x7fffba400000 access=0x8000000000000004 current=makedumpfile
> > > > [   44.337663] hash-mmu:     trap=0x300 vsid=0x13a109c ssize=1 base psize=2 psize 2 pte=0xc000000050000504
> > > > [   44.337677] hash-mmu: mm: Hashing failure ! EA=0x7fffba400000 access=0x8000000000000004 current=makedumpfile
> > > > [   44.337692] hash-mmu:     trap=0x300 vsid=0x13a109c ssize=1 base psize=2 psize 2 pte=0xc000000050000504
> > > > [   44.337708] makedumpfile[469]: unhandled signal 7 at 00007fffba400000 nip 00007fffbbc4d7fc lr 000000011356ca3c code 2
> > > > [   44.338548] Core dump to |/bin/false pipe failed
> > > > /lib/kdump-lib-initramfs.sh: line 98:   469 Bus error               $CORE_COLLECTOR /proc/vmcore $_mp/$KDUMP_PATH/$HOST_IP-$DATEDIR/vmcore-incomplete
> > > > kdump: saving vmcore failed
> > > >
> > > > * Root cause *
> > > >    After analyzing, it turns out that in the current implementation,
> > > > when hot-removing lmb, the KOBJ_REMOVE event ejects before the dt updating as
> > > > the code __remove_memory() comes before drmem_update_dt().
> > > > So in kdump kernel, when read_from_oldmem() resorts to
> > > > pSeries_lpar_hpte_insert() to install hpte, but fails with -2 due to
> > > > non-exist pfn. And finally, low_hash_fault() raise SIGBUS to process, as it
> > > > can be observed "Bus error"
> > > >
> > > >  From a viewpoint of listener and publisher, the publisher notifies the
> > > > listener before data is ready.  This introduces a problem where udev
> > > > launches kexec-tools (due to KOBJ_REMOVE) and loads a stale dt before
> > > > updating. And in capture kernel, makedumpfile will access the memory based
> > > > on the stale dt info, and hit a SIGBUS error due to an un-existed lmb.
> > > >
> > > > * Fix *
> > > > This bug is introduced by commit 063b8b1251fd
> > > > ("powerpc/pseries/memory-hotplug: Only update DT once per memory DLPAR
> > > > request"), which tried to combine all the dt updating into one.
> > > >
> > > > To fix this issue, meanwhile not to introduce a quadratic runtime
> > > > complexity by the model:
> > > >    dlpar_memory_add_by_count
> > > >      for_each_drmem_lmb             <--
> > > >        dlpar_add_lmb
> > > >          drmem_update_dt(_v1|_v2)
> > > >            for_each_drmem_lmb       <--
> > > > The dt should still be only updated once, and just before the last memory
> > > > online/offline event is ejected to user space. Achieve this by tracing the
> > > > num of lmb added or removed.
> > > >
> > > > Signed-off-by: Pingfan Liu <kernelfans@gmail.com>
> > > > Cc: Michael Ellerman <mpe@ellerman.id.au>
> > > > Cc: Hari Bathini <hbathini@linux.ibm.com>
> > > > Cc: Nathan Lynch <nathanl@linux.ibm.com>
> > > > Cc: Nathan Fontenot <nfont@linux.vnet.ibm.com>
> > > > Cc: Laurent Dufour <ldufour@linux.ibm.com>
> > > > To: linuxppc-dev@lists.ozlabs.org
> > > > Cc: kexec@lists.infradead.org
> > > > ---
> > > > v4 -> v5: change dlpar_add_lmb()/dlpar_remove_lmb() prototype to report
> > > >            whether dt is updated successfully.
> > > >            Fix a condition boundary check bug
> > > > v3 -> v4: resolve a quadratic runtime complexity issue.
> > > >            This series is applied on next-test branch
> > > >   arch/powerpc/platforms/pseries/hotplug-memory.c | 102 +++++++++++++++++++-----
> > > >   1 file changed, 80 insertions(+), 22 deletions(-)
> > > >
> > > > diff --git a/arch/powerpc/platforms/pseries/hotplug-memory.c b/arch/powerpc/platforms/pseries/hotplug-memory.c
> > > > index 46cbcd1..1567d9f 100644
> > > > --- a/arch/powerpc/platforms/pseries/hotplug-memory.c
> > > > +++ b/arch/powerpc/platforms/pseries/hotplug-memory.c
> > > > @@ -350,13 +350,22 @@ static bool lmb_is_removable(struct drmem_lmb *lmb)
> > > >       return true;
> > > >   }
> > > >
> > > > -static int dlpar_add_lmb(struct drmem_lmb *);
> > > > +enum dt_update_status {
> > > > +     DT_NOUPDATE,
> > > > +     DT_TOUPDATE,
> > > > +     DT_UPDATED,
> > > > +};
> > > > +
> > > > +/* "*dt_update" returns DT_UPDATED if updated */
> > > > +static int dlpar_add_lmb(struct drmem_lmb *lmb,
> > > > +             enum dt_update_status *dt_update);
> > > >
> > > > -static int dlpar_remove_lmb(struct drmem_lmb *lmb)
> > > > +static int dlpar_remove_lmb(struct drmem_lmb *lmb,
> > > > +             enum dt_update_status *dt_update)
> > > >   {
> > > >       unsigned long block_sz;
> > > >       phys_addr_t base_addr;
> > > > -     int rc, nid;
> > > > +     int rc, ret, nid;
> > > >
> > > >       if (!lmb_is_removable(lmb))
> > > >               return -EINVAL;
> > > > @@ -372,6 +381,13 @@ static int dlpar_remove_lmb(struct drmem_lmb *lmb)
> > > >       invalidate_lmb_associativity_index(lmb);
> > > >       lmb_clear_nid(lmb);
> > > >       lmb->flags &= ~DRCONF_MEM_ASSIGNED;
> > > > +     if (*dt_update) {
> > Original, I plan to use it to exclude the case of DT_NOUPDATE, which is value 0.
> > And I think it looks better by using if (*dt_update == DT_TOUPDATE)
> > >
> > > That test is wrong, you should do:
> > >          if (*dt_update && *dt_update == DT_TOUPDATE) {
> > I think you mean  if (dt_update && *dt_update == DT_TOUPDATE) {
> > >
> > > With the current code, the device tree is updated all the time.
> > >
> > > Another option would be to pass a valid pointer (!= NULL) only when DT update is
> > > required, this way you don't need the DT_TOUPDATE value. The caller would have
> > > to set the pointer accordingly. The advantage with this option is the caller is
> > > guaranteed that its variable is not touched by the callee when no device tree is
> > > requested. A simple boolean pointer would be enough without the need to this enum.
> > It is expected that dlpar_remove_lmb/dlpar_add_lmb can report whether
> > they successfully update dt or not. So the caller can handle the
> > different cases.
>
> Is there any plan to refresh this patch to apply to master?
>
Spreading the failure and rollback logic around the code looks urgly.
Even myself dislike it. But I have not found a better way out.

> I am using an older revision of this patch so I am not in the position
> to repost an updated version.
>
> I lack some otimization in my patch so I probably have the quadratic
> coplexity of the add mentioned above.
>
Do you have any good ideas?

Thanks,
Pingfan