Re: [PATCH] crash: keep crashkernel CMA pages reserved if activation fails

[Usama Arif <usama.arif@linux.dev>]

On Fri, 6 Nov 2026 12:21:12 +0100 Mikhail Zaslonko <zaslonko@linux.ibm.com> wrote:

> 
> 
> On 04-Nov-26 15:56, Usama Arif wrote:
> > reserve_crashkernel_cma() adds each crashkernel CMA area to
> > crashk_cma_ranges[] as soon as it is declared. If the area later fails
> > activation, e.g. because movablecore= put the ZONE_MOVABLE boundary
> > inside it, cma_activate_area() releases the pages to the buddy allocator
> > as ordinary memory. The range is still handed to the crash kernel and
> > excluded from the vmcore, so kernel data that lands there is overwritten
> > by the crash kernel and missing from the dump.
> > 
> > Mark the areas with cma_reserve_pages_on_error(), so that a failed
> > activation leaves the pages reserved: lost to the first kernel, but safe
> > for the crash kernel to use.
> > 
> > Fixes: ab475510e042 ("kdump: implement reserve_crashkernel_cma")
> > Signed-off-by: Usama Arif <usama.arif@linux.dev>
> > ---
> s390 hits this without movablecore= (adding the s390 maintainers and
> linux-s390) since ZONE_DMA ends at 2G here.
> CMA tries to reserve areas above 4G first, but falls back to lower
> memory when it doesn't fit, so a cma area can end up across the 2G boundary.
> I ran into this while adding crashkernel=,cma support for s390, which I
> will post soon, and came up with the same fix.
> 
> About dropping the range instead, as you offered for v2: I'd keep this
> version. With the range dropped, the kdump kernel gets less memory than
> configured, and that only shows at crash time. With this patch, the
> the first kernel 'loses' the memory as with a regular crashkernel= reservation
> of the same size.

Yes, I do agree with this. Losing memory in first kernel is preferrable
and it will then force someone to fix whatever is broken rather than
finding out when their kernel crashes.

> 
> There is a second problem in the same loop: all areas are named
> "crashkernel", which breaks the CMA sysfs and debugfs entries when
> there are several. I've sent a separate fix for that:
> 
> https://lore.kernel.org/r/20261106104309.3669583-1-zaslonko@linux.ibm.com
> 

Thanks! I reviewed this now.

> Reviewed-by: Mikhail Zaslonko <zaslonko@linux.ibm.com>
> Tested-by: Mikhail Zaslonko <zaslonko@linux.ibm.com>
> 

Thanks for testing and reviewing!

Usama