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 03:39pm, David Hildenbrand wrote:
> On 20.11.24 15:05, Baoquan He wrote:
> > 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
> > config NEED_PROC_VMCORE_DEVICE_RAM
> >          def y
> > 
> > arch/s390/Kconfig:
> > config NEED_PROC_VMCORE_DEVICE_RAM
> >          def y
> > ==================================
> 
> That would work, but I don't completely like it.
> 
> (a) I want s390x to select NEED_PROC_VMCORE_DEVICE_RAM instead. Staring at a
> bunch of similar cases (git grep "config NEED" | grep Kconfig, git grep
> "config ARCH_WANTS" | grep Kconfig), "select" is the common way to do it.
> 
> So unless there is a pretty good reason, I'll keep
> NEED_PROC_VMCORE_DEVICE_RAM as is.

That's easy to satify, see below:

============simple version=====
fs/proc/Kconfig:
config NEED_PROC_VMCORE_DEVICE_RAM
        def n

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 S390
        select NEED_PROC_VMCORE_DEVICE_RAM
==============================

> 
> (b) In the context of this patch, "depends on VIRTIO_MEM" does not make
> sense. We could have an intermediate:
> 
> config PROC_VMCORE_DEVICE_RAM
>          def_bool n
>          depends on PROC_VMCORE
>          depends on NEED_PROC_VMCORE_DEVICE_RAM
> 
> And change that with VIRTIO_MEM support in the relevant patch.

Oh, it's not comment for this patch, I made the simple version based on
the whole patchset. When I had a glance at this patch, I also took
several iterations to get it after I applied the whole patchset and
tried to understand the whole code.

> 
> 
> I faintly remember that we try avoiding such dependencies and prefer
> selecting Kconfigs instead. Just look at the SPLIT_PTE_PTLOCKS mess we still
> have to clean up. But as we don't expect that many providers for now, I
> don't care.

With the simple version, Kconfig learner as me can easily understand what
they are doing. If it took you a couple of iterations to make them as
you had mentioned earlier, and it took me several iterations to
understand them, I believe there must be room to improve the presented
ones in this patchset. These are only my humble opinion, and I am not
aware of virtio-mem at all, I'll leave this to you and other virtio-mem
dev to decide what should be taken. Thanks for your patience and
provided information, I learned a lot from this discussion.

===================
fs/proc/Kconfig:
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

drivers/virtio/Kconfig:
config VIRTIO_MEM
        select PROVIDE_PROC_VMCORE_DEVICE_RAM if PROC_VMCORE
                                              ~~~~~~~~~~~~~~

arch/s390/Kconfig:
config S390
        select NEED_PROC_VMCORE_DEVICE_RAM if PROC_VMCORE
                                           ~~~~~~~~~~~~~~
========================

One last thing I haven't got well, If PROC_VMCORE_DEVICE_RAM has had
dependency on PROC_VMCORE, can we take off the ' if PROC_VMCORE' when
select PROVIDE_PROC_VMCORE_DEVICE_RAM and NEED_PROC_VMCORE_DEVICE_RAM?

Thanks
Baoquan