Re: [PATCH] ppc/fadump: collect dump if the collected size is lesser than reserved

[Shivang Upadhyay <shivangu@linux.ibm.com>]

On Sun, 2026-07-19 at 12:39 +0530, Sourabh Jain wrote:
> Could you please reword the commit title to make it a bit clearer?
> 
> 
> On 14/07/26 23:00, Shivang Upadhyay wrote:
> > When a machine is subjected to CPUs add/remove, using dlpar
> > operations, the number of collected CPU_NOTES can change.
> > As per PAPR, collected dump size should not be more than
> > allocated size. Reflecting the same in source.
> 
> Can you add more details about the problem you are trying to solve
> with this patch and how.
> 
> Adding the error message and scenario would be really helpful in
> understanding the problem.
> 

Hi Sourabh,

 When a qemu ppc machine is booted with fadump=on and `-
smp=x,maxcpus=y`, on the panic kernel, /proc/vmcore is not generated
because dump_bytes and source_len does'nt match for CPU_STATE_DATA
region in fadump, then we just give up on parsing rest of the data.


> Can you add Closes tag if it is reported upstream and if possible
> fixes 
> tag too.
> 
> > Signed-off-by: Shivang Upadhyay <shivangu@linux.ibm.com>
> > ---
> >   arch/powerpc/platforms/pseries/rtas-fadump.c | 3 ++-
> >   1 file changed, 2 insertions(+), 1 deletion(-)
> > 
> > diff --git a/arch/powerpc/platforms/pseries/rtas-fadump.c
> > b/arch/powerpc/platforms/pseries/rtas-fadump.c
> > index 3bb4ac2ab6cc..19a5adaf326b 100644
> > --- a/arch/powerpc/platforms/pseries/rtas-fadump.c
> > +++ b/arch/powerpc/platforms/pseries/rtas-fadump.c
> > @@ -469,7 +469,8 @@ static int __init rtas_fadump_process(struct
> > fw_dump *fadump_conf)
> >   				pr_err("Dump taken by platform is
> > not valid (%d)\n", i);
> >   				rc = -EINVAL;
> >   			}
> > -			if (fdm_active->rgn[i].bytes_dumped !=
> > fdm_active->rgn[i].source_len) {
> > +			if (be64_to_cpu(fdm_active-
> > >rgn[i].bytes_dumped)
> > +			    > be64_to_cpu(fdm_active-
> > >rgn[i].source_len)) {
> 
> Can you please share your observations about `bytes_dump` for both
> QEMU 
> and a
> real system (LPAR) where the number of online CPUs is not equal to
> the 
> maximum

I have the following observation.

I booted LPAR with 8 cpus. After crashing it I saw that fadump
CPU_STATE_DATA has notes for total 16 cpus, and only top 8 notes have
valid entries. when trying with 16, I see all NOTES have entried
filled.

I previously proposed this fake entries fix in qemu [1]. But that is
not compliant with PAPR, which states that cpu notes should be
collected for current_cpus only. 

As per current states of things, reservation is always done for
maxcpus, (LPAR and QEMU both), NOTES collections is done for current
cpus on QEMU, and maxcpus on LPARs.

My proposal here is that we make the check for bytes_dumped and
source_len, less restrictive to let QEMU also generate /proc/vmcore,
when current_cpus != maxcpus.


Regards 
~Shivang.

[1]
https://lore.kernel.org/qemu-devel/20260520092857.1079193-1-shivangu@linux.ibm.com/