Re: [PATCH v4] arm64: Enable vmalloc-huge with ptdump

[Dev Jain <dev.jain@arm.com>]

On 26/06/25 10:55 am, Dev Jain wrote:
> arm64 disables vmalloc-huge when kernel page table dumping is enabled,
> because an intermediate table may be removed, potentially causing the
> ptdump code to dereference an invalid address. We want to be able to
> analyze block vs page mappings for kernel mappings with ptdump, so to
> enable vmalloc-huge with ptdump, synchronize between page table removal in
> pmd_free_pte_page()/pud_free_pmd_page() and ptdump pagetable walking. We
> use mmap_read_lock and not write lock because we don't need to synchronize
> between two different vm_structs; two vmalloc objects running this same
> code path will point to different page tables, hence there is no race.
>
> For pud_free_pmd_page(), we isolate the PMD table to avoid taking the lock
> 512 times again via pmd_free_pte_page().
>
> We implement the locking mechanism using static keys, since the chance
> of a race is very small. Observe that the synchronization is needed
> to avoid the following race:
>
> CPU1							CPU2
> 						take reference of PMD table
> pud_clear()
> pte_free_kernel()
> 						walk freed PMD table
>
> and similar race between pmd_free_pte_page and ptdump_walk_pgd.
>
> Therefore, there are two cases: if ptdump sees the cleared PUD, then
> we are safe. If not, then the patched-in read and write locks help us
> avoid the race.
>
> To implement the mechanism, we need the static key access from mmu.c and
> ptdump.c. Note that in case !CONFIG_PTDUMP_DEBUGFS, ptdump.o won't be a
> target in the Makefile, therefore we cannot initialize the key there, as
> is being done, for example, in the static key implementation of
> hugetlb-vmemmap. Therefore, include asm/cpufeature.h, which includes
> the jump_label mechanism. Declare the key there and define the key to false
> in mmu.c.
>
> No issues were observed with mm-selftests. No issues were observed while
> parallelly running test_vmalloc.sh and dumping the kernel pagetable through
> sysfs in a loop.
>
> Reviewed-by: Ryan Roberts <ryan.roberts@arm.com>
> Signed-off-by: Dev Jain <dev.jain@arm.com>
> ---

Sorry, I observed that post v3 I forgot to add the change wherein
I drop the PTDUMP condition from arch_vmap_pxd_supported, I'll add
that back in the next version. Thanks Anshuman for the spot.