Re: [PATCH v1 07/11] fs/proc/vmcore: introduce PROC_VMCORE_DEVICE_RAM to detect device RAM ranges in 2nd kernel

[Baoquan He <bhe@redhat.com>]

On 11/20/24 at 11:48am, David Hildenbrand wrote:
> On 20.11.24 11:13, Baoquan He wrote:
> > On 10/25/24 at 05:11pm, David Hildenbrand wrote:
> > > s390 allocates+prepares the elfcore hdr in the dump (2nd) kernel, not in
> > > the crashed kernel.
> > > 
> > > RAM provided by memory devices such as virtio-mem can only be detected
> > > using the device driver; when vmcore_init() is called, these device
> > > drivers are usually not loaded yet, or the devices did not get probed
> > > yet. Consequently, on s390 these RAM ranges will not be included in
> > > the crash dump, which makes the dump partially corrupt and is
> > > unfortunate.
> > > 
> > > Instead of deferring the vmcore_init() call, to an (unclear?) later point,
> > > let's reuse the vmcore_cb infrastructure to obtain device RAM ranges as
> > > the device drivers probe the device and get access to this information.
> > > 
> > > Then, we'll add these ranges to the vmcore, adding more PT_LOAD
> > > entries and updating the offsets+vmcore size.
> > > 
> > > Use Kconfig tricks to include this code automatically only if (a) there is
> > > a device driver compiled that implements the callback
> > > (PROVIDE_PROC_VMCORE_DEVICE_RAM) and; (b) the architecture actually needs
> > > this information (NEED_PROC_VMCORE_DEVICE_RAM).
> > > 
> > > The current target use case is s390, which only creates an elf64
> > > elfcore, so focusing on elf64 is sufficient.
> > > 
> > > Signed-off-by: David Hildenbrand <david@redhat.com>
> > > ---
> > >   fs/proc/Kconfig            |  25 ++++++
> > >   fs/proc/vmcore.c           | 156 +++++++++++++++++++++++++++++++++++++
> > >   include/linux/crash_dump.h |   9 +++
> > >   3 files changed, 190 insertions(+)
> > > 
> > > diff --git a/fs/proc/Kconfig b/fs/proc/Kconfig
> > > index d80a1431ef7b..1e11de5f9380 100644
> > > --- a/fs/proc/Kconfig
> > > +++ b/fs/proc/Kconfig
> > > @@ -61,6 +61,31 @@ config PROC_VMCORE_DEVICE_DUMP
> > >   	  as ELF notes to /proc/vmcore. You can still disable device
> > >   	  dump using the kernel command line option 'novmcoredd'.
> > > +config PROVIDE_PROC_VMCORE_DEVICE_RAM
> > > +	def_bool n
> > > +
> > > +config NEED_PROC_VMCORE_DEVICE_RAM
> > > +	def_bool n
> > > +
> > > +config PROC_VMCORE_DEVICE_RAM
> > > +	def_bool y
> > > +	depends on PROC_VMCORE
> > > +	depends on NEED_PROC_VMCORE_DEVICE_RAM
> > > +	depends on PROVIDE_PROC_VMCORE_DEVICE_RAM
> > 
> > Kconfig item is always a thing I need learn to master.
> 
> Yes, it's usually a struggle to get it right. It took me a couple of
> iterations to get to this point :)
> 
> > When I checked
> > this part, I have to write them down to deliberate. I am wondering if
> > below 'simple version' works too and more understandable. Please help
> > point out what I have missed.
> > 
> > ===========simple version======
> > config PROC_VMCORE_DEVICE_RAM
> >          def_bool y
> >          depends on PROC_VMCORE && VIRTIO_MEM
> >          depends on NEED_PROC_VMCORE_DEVICE_RAM
> > 
> > config S390
> >          select NEED_PROC_VMCORE_DEVICE_RAM
> > ============

Sorry, things written down didn't correctly reflect them in my mind. 

===========simple version======
fs/proc/Kconfig:
config PROC_VMCORE_DEVICE_RAM
        def_bool y
        depends on PROC_VMCORE && VIRTIO_MEM
        depends on NEED_PROC_VMCORE_DEVICE_RAM

arch/s390/Kconfig:
config NEED_PROC_VMCORE_DEVICE_RAM
        def y
==================================


> 
> So the three changes you did are
> 
> (a) Remove the config option but select/depend on them.
> 
> (b) Remove the "depends on PROC_VMCORE" from PROC_VMCORE_DEVICE_RAM,
>     and the "if PROC_VMCORE" from s390.
> 
> (c) Remove the PROVIDE_PROC_VMCORE_DEVICE_RAM
> 
> 
> Regarding (a), that doesn't work. If you select a config option that doesn't
> exist, it is silently dropped. It's always treated as if it wouldn't be set.
> 
> Regarding (b), I think that's an anti-pattern (having config options enabled
> that are completely ineffective) and I don't see a benefit dropping them.
> 
> Regarding (c), it would mean that s390x unconditionally includes that code
> even if virtio-mem is not configured in.
> 
> So while we could drop PROVIDE_PROC_VMCORE_DEVICE_RAM -- (c), it would that
> we end up including code in configurations that don't possibly need it.
> That's why I included that part.
> 
> > 
> > 
> > ======= config items extracted from this patchset====
> > config PROVIDE_PROC_VMCORE_DEVICE_RAM
> >          def_bool n
> > 
> > config NEED_PROC_VMCORE_DEVICE_RAM
> >          def_bool n
> > 
> > config PROC_VMCORE_DEVICE_RAM
> >          def_bool y
> >          depends on PROC_VMCORE
> >          depends on NEED_PROC_VMCORE_DEVICE_RAM
> >          depends on PROVIDE_PROC_VMCORE_DEVICE_RAM
> > 
> > config VIRTIO_MEM
> > 	depends on X86_64 || ARM64 || RISCV
> >           ~~~~~ I don't get why VIRTIO_MEM dones't depend on S390 if
> >                 s390 need PROC_VMCORE_DEVICE_RAM.
> 
> This series depends on s390 support for virtio-mem, which just went
> upstream.

Got It, I just applied this series on top of the latest mainline's
master branch. Thanks for telling.

> 
> 
> commit 38968bcdcc1d46f2fdcd3a72599d5193bf8baf84
> Author: David Hildenbrand <david@redhat.com>
> Date:   Fri Oct 25 16:14:49 2024 +0200
> 
>     virtio-mem: s390 support
> 
> 
> >          ......
> >          select PROVIDE_PROC_VMCORE_DEVICE_RAM if PROC_VMCORE
> > 
> > config S390
> >          select NEED_PROC_VMCORE_DEVICE_RAM if PROC_VMCORE
> > =================================================
> > 
> 
> Thanks for having a look!
> 
> -- 
> Cheers,
> 
> David / dhildenb
>