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

[Bradley Morgan]

On 4 November 2026 21:52:36 GMT, Usama Arif <usama.arif@xxxxxxxxx> wrote:
>On Wed, 04 Nov 2026 19:16:42 +0000 Bradley Morgan <brads@xxxxxxxxxxxxxx>
>wrote:
>
>> On 4 November 2026 14:56:00 GMT, Usama Arif <usama.arif@xxxxxxxxx>
>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@xxxxxxxxx>
>> >---
>> > kernel/crash_reserve.c | 1 +
>> > 1 file changed, 1 insertion(+)
>> >
>> >diff --git a/kernel/crash_reserve.c b/kernel/crash_reserve.c
>> >index a03f0a4305a4e..a4b616ed49482 100644
>> >--- a/kernel/crash_reserve.c
>> >+++ b/kernel/crash_reserve.c
>> >@@ -498,6 +498,7 @@ void __init reserve_crashkernel_cma(unsigned long
>long cma_size)
>> > 			continue;
>> > 		}
>> > 
>> >+		cma_reserve_pages_on_error(res);
>> 
>> You do know that this will activate under every bitmap_zalloc() failure,
>> right? Is that accounted for?
>
>Yes, that's intended. By the time cma_activate_area() runs, the range is
>already in crashk_cma_ranges[], so the crash kernel will use it no matter
>why activation failed. Keeping the pages reserved is the only way to make
>sure the first kernel has nothing there that gets overwritten or goes
>missing from the vmcore; the area then behaves like a regular
>crashkernel= reservation of the same size.
>
>fadump made the same choice; it is what cma_reserve_pages_on_error() was
>added for in commit 27d121d0ec6d ("mm/cma: provide option to opt out
>from exposing pages on activation failure").
>
>The bitmap is one bit per page (32 KiB per GiB with 4K pages) and is
>allocated at core_initcall, so a failure there is very unlikely.
>
>The alternative is to give the memory back and to drop the
>range from crashk_cma_ranges[] when activation fails, leaving the crash
>kernel with less memory than requested. I do think that is acceptable as
>well. But we do need either whats done in this patch or give the memory
>back. Happy to send the alternative as v2 is everyone thinks that the
>better way forward.

Hmm. No approach at the moment is the "perfect" approach so far.

>
>> 
>> > 		crashk_cma_ranges[crashk_cma_cnt].start = cma_get_base(res);
>> > 		crashk_cma_ranges[crashk_cma_cnt].end =
>> > 			crashk_cma_ranges[crashk_cma_cnt].start +
>> >
>> 
>> --- Thanks!
>> 

--- Thanks!