linux-kernel - Re: [PATCH v3] KHO: Fix boot failure due to kmemleak access to non-PRESENT pages

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Message-ID: <mafs0ms4bzhng.fsf@kernel.org>
Date: Mon, 24 Nov 2025 14:25:55 +0100
From: Pratyush Yadav <pratyush@...nel.org>
To: Pratyush Yadav <pratyush@...nel.org>
Cc: ranxiaokai627@....com,  rppt@...nel.org,  akpm@...ux-foundation.org,
  catalin.marinas@....com,  changyuanl@...gle.com,  graf@...zon.com,
  pasha.tatashin@...een.com,  kexec@...ts.infradead.org,
  linux-kernel@...r.kernel.org,  linux-mm@...ck.org,
  ran.xiaokai@....com.cn
Subject: Re: [PATCH v3] KHO: Fix boot failure due to kmemleak access to
 non-PRESENT pages

On Mon, Nov 24 2025, Pratyush Yadav wrote:

> On Mon, Nov 24 2025, ranxiaokai627@....com wrote:
>
>> From: Ran Xiaokai <ran.xiaokai@....com.cn>
>>
>> When booting with debug_pagealloc=on while having:
>> CONFIG_KEXEC_HANDOVER_ENABLE_DEFAULT=y
>> CONFIG_DEBUG_KMEMLEAK_DEFAULT_OFF=n
>> the system fails to boot due to page faults during kmemleak scanning.
>>
>> This occurs because:
>> With debug_pagealloc is enabled, __free_pages() invokes
>> debug_pagealloc_unmap_pages(), clearing the _PAGE_PRESENT bit for
>> freed pages in the kernel page table.
>> KHO scratch areas are allocated from memblock and noted by kmemleak. But
>> these areas don't remain reserved but released later to the page allocator
>> using init_cma_reserved_pageblock(). This causes subsequent kmemleak scans
>> access non-PRESENT pages, leading to fatal page faults.
>>
>> Mark scratch areas with kmemleak_ignore_phys() after they are allocated
>> from memblock to exclude them from kmemleak scanning before they are
>> released to buddy allocator to fix this.
>>
>> Fixes: 3dc92c311498 ("kexec: add Kexec HandOver (KHO) generation helpers")
>> Signed-off-by: Ran Xiaokai <ran.xiaokai@....com.cn>
>> Reviewed-by: Mike Rapoport (Microsoft) <rppt@...nel.org>
>> ---
>>  kernel/liveupdate/kexec_handover.c | 2 ++
>>  1 file changed, 2 insertions(+)
>>
>> diff --git a/kernel/liveupdate/kexec_handover.c b/kernel/liveupdate/kexec_handover.c
>> index 224bdf5becb6..c729d455ee7b 100644
>> --- a/kernel/liveupdate/kexec_handover.c
>> +++ b/kernel/liveupdate/kexec_handover.c
>> @@ -11,6 +11,7 @@
>>  
>>  #include <linux/cleanup.h>
>>  #include <linux/cma.h>
>> +#include <linux/kmemleak.h>
>>  #include <linux/count_zeros.h>
>>  #include <linux/kexec.h>
>>  #include <linux/kexec_handover.h>
>> @@ -1369,6 +1370,7 @@ static __init int kho_init(void)
>>  		unsigned long count = kho_scratch[i].size >> PAGE_SHIFT;
>>  		unsigned long pfn;
>>  
>> +		kmemleak_ignore_phys(kho_scratch[i].addr);
>
> Can you please put the explanation you gave in [0] for why this is not
> necessary in KHO boot as a comment here?

And also an explanation of why this is necessary in the first place. You
do explain that in the commit message, but a shorter version as a
comment will make this a lot easier to understand instead of having to
dig through git history.

[...]

-- 
Regards,
Pratyush Yadav