Re: [PATCH v4] arm64: kdump: simplify the reservation behaviour of crashkernel=,high

[Baoquan He <bhe@redhat.com>]

Hi Leizhen,

On 03/24/23 at 10:47am, Leizhen (ThunderTown) wrote:
...... 
> >>>> 2) with the fixed CRASH_ADDR_LOW_MAX as 4G, we can easily fix the
> >>>> problem of base page mapping for the whole linear mapping if crsahkernel=
> >>>> is set in kernel parameter shown in [1] at bottom. 
> >>>
> >>> That's a different problem ;). I should re-read that thread, forgot most
> >>> of the details but I recall one of the counter arguments was that there
> >>> isn't a strong case to unmap the crashkernel reservation. Now, if we
> >>> place crashdump kernel image goes in the 'high' reservation, can we not
> >>> leave the 'low' reservation mapped? We don't really care about it as it
> >>> wouldn't have any meaningful code/data to be preserved. If the 'high'
> >>> one goes above 4G always, we don't depend on the arm64_dma_phys_limit.
> >>
> >> Yes, this looks ideal. While it only works when crashkernel=,high case and
> >> it succeeds to reserve a memory region for the specified size of crashkernel
> >> high memory. At below, we have 4 cases of crashkernel= syntax:
> >>
> >> crashkernel=size
> >> 1)first attempt:  low memory under arm64_dma_phys_limit
> >> 2)fallback:       finding memory above 4G
> > 
> > (2) should be 'finding memory above arm64_dma_phys_limit' to keep the
> > current behaviour for RPi4.
> > 
> >> crashkernel=size,high
> >> 3)first attempt:  finding memory above 4G
> >> 4)fallback:       low memory under arm64_dma_phys_limit
> > 
> > Yes.
> > 
> >> case 3) works with your suggestion. However, 1), 2), 4) all need to
> >> defer to bootmem_init(). With these cases and different handling,
> >> reserve_crashkernel() could be too complicated.
> > 
> > Ah, because of the fallback below arm64_dma_phys_limit as in (4), we
> > still can't move the full crashkernel reservation early. Well, we could
> > do it in two steps: (a) early attempt at crashkernel reservation above
> > 4G if 'high' was specified and we avoid mapping it if successful and (b)
> > do the late crashkernel reservation below arm64_dma_phys_limit and skip
> > unmapping as being too late. This way most server-like platforms would
> > get a reservation above 4G, unmapped.
> > 
> >> I am wondering if we can cancel the protection of crashkernel memory
> >> region on arm64 for now. In earlier discussion, people questioned if the
> >> protection is necessary on arm64. After comparison, I would rather take
> >> away the protection method of crashkernel region since they try to
> >> protect in a chance in one million , while the base page mapping for the
> >> whole linear mapping is mitigating arm64 high end server always.
> > 
> > This works for me. We can add the protection later for addresses above
> > 4GB only as mentioned above.
> 
> Recently, I've also been rethinking the performance issues when kdump is
> enabled. I have a new idea. For crashkernel=X, we can temporarily search
> for free memory from the low address to the high address. As below:
> 
> save_bottom_up = memblock_bottom_up();
> if (!high)
> 	memblock_set_bottom_up(true);
> crash_base = memblock_phys_alloc_range(crash_size, CRASH_ALIGN, crash_base, crash_max);
> memblock_set_bottom_up(save_bottom_up);
> 
> The final code change should be small, and I'll try it today.

I have sent a patchset to remove the crashkernel region protection code
as per Catalin's confirmation. I personally like the code conciseness w/o
protection because kinds of crahskernel reservation has been complex,
the situation on arm64 will makes it worse if we try to keep the
protection and fix the performance issue. While I am glad to see any
attempt to achieve the two goals if it's satisfactory.


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