Re: [PATCH V2] mm/ptdump: Take the memory hotplug lock inside ptdump_walk_pgd()

[Dev Jain]

On 20/06/25 10:54 am, Anshuman Khandual wrote:
> Memory hot remove unmaps and tears down various kernel page table regions
> as required. The ptdump code can race with concurrent modifications of the
> kernel page tables. When leaf entries are modified concurrently, the dump
> code may log stale or inconsistent information for a VA range, but this is
> otherwise not harmful.
>
> But when intermediate levels of kernel page table are freed, the dump code
> will continue to use memory that has been freed and potentially reallocated
> for another purpose. In such cases, the ptdump code may dereference bogus
> addresses, leading to a number of potential problems.
>
> To avoid the above mentioned race condition, platforms such as arm64, riscv
> and s390 take memory hotplug lock, while dumping kernel page table via the
> sysfs interface /sys/kernel/debug/kernel_page_tables.
>
> Similar race condition exists while checking for pages that might have been
> marked W+X via /sys/kernel/debug/kernel_page_tables/check_wx_pages which in
> turn calls ptdump_check_wx(). Instead of solving this race condition again,
> let's just move the memory hotplug lock inside generic ptdump_check_wx()
> which will benefit both the scenarios.
>
> Drop get_online_mems() and put_online_mems() combination from all existing
> platform ptdump code paths.
>
> Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
> Cc: Will Deacon <will@xxxxxxxxxx>
> Cc: Ryan Roberts <ryan.roberts@xxxxxxx>
> Cc: Paul Walmsley <paul.walmsley@xxxxxxxxxx>
> Cc: Palmer Dabbelt <palmer@xxxxxxxxxxx>
> Cc: Alexander Gordeev <agordeev@xxxxxxxxxxxxx>
> Cc: Gerald Schaefer <gerald.schaefer@xxxxxxxxxxxxx>
> Cc: Heiko Carstens <hca@xxxxxxxxxxxxx>
> Cc: Vasily Gorbik <gor@xxxxxxxxxxxxx>
> Cc: Christian Borntraeger <borntraeger@xxxxxxxxxxxxx>
> Cc: Sven Schnelle <svens@xxxxxxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> CC: linux-arm-kernel@xxxxxxxxxxxxxxxxxxx
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> Cc: linux-riscv@xxxxxxxxxxxxxxxxxxx
> Cc: linux-s390@xxxxxxxxxxxxxxx
> Cc: linux-mm@xxxxxxxxx
> Signed-off-by: Anshuman Khandual <anshuman.khandual@xxxxxxx>
> ---

Reviewed-by: Dev Jain <dev.jain@xxxxxxx>