Re: [RESEND PATCH v5] arm64: Enable vmalloc-huge with ptdump

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

On 16/09/25 4:00 pm, Will Deacon wrote:
> Hi Dev,
>
> On Wed, Jul 23, 2025 at 09:48:27PM +0530, Dev Jain wrote:
>> @@ -1301,16 +1319,76 @@ int pud_free_pmd_page(pud_t *pudp, unsigned long addr)
>>   	}
>>   
>>   	table = pmd_offset(pudp, addr);
>> +
>> +	/*
>> +	 * Our objective is to prevent ptdump from reading a PMD table which has
>> +	 * been freed.  Assume that ptdump_walk_pgd() (call this thread T1)
>> +	 * executes completely on CPU1 and pud_free_pmd_page() (call this thread
>> +	 * T2) executes completely on CPU2. Let the region sandwiched by the
>> +	 * mmap_write_lock/unlock in T1 be called CS (the critical section).
>> +	 *
>> +	 * Claim: The CS of T1 will never operate on a freed PMD table.
>> +	 *
>> +	 * Proof:
>> +	 *
>> +	 * Case 1: The static branch is visible to T2.
>> +	 *
>> +	 * Case 1 (a): T1 acquires the lock before T2 can.
>> +	 * T2 will block until T1 drops the lock, so pmd_free() will only be
>> +	 * executed after T1 exits CS.
>> +	 *
>> +	 * Case 1 (b): T2 acquires the lock before T1 can.
>> +	 * The sequence of barriers issued in __flush_tlb_kernel_pgtable()
>> +	 * ensures that an empty PUD (via pud_clear()) is visible to T1 before
>> +	 * T1 can enter CS, therefore it is impossible for the CS to get hold
>> +	 * of the address of the isolated PMD table.
>> +	 *
>> +	 * Case 2: The static branch is not visible to T2.
>> +	 *
>> +	 * Since static_branch_enable() (via dmb(ish)) and mmap_write_lock()
>> +	 * have acquire semantics, it is guaranteed that the static branch
>> +	 * will be visible to all CPUs before T1 can enter CS. The static
>> +	 * branch not being visible to T2 therefore guarantees that T1 has
>> +	 * not yet entered CS .... (i)
>> +	 * The sequence of barriers via __flush_tlb_kernel_pgtable() in T2
>> +	 * implies that if the invisibility of the static branch has been
>> +	 * observed by T2 (i.e static_branch_unlikely() is observed as false),
>> +	 * then all CPUs will have observed an empty PUD ... (ii)
>> +	 * Combining (i) and (ii), we conclude that T1 observes an empty PUD
>> +	 * before entering CS => it is impossible for the CS to get hold of
>> +	 * the address of the isolated PMD table. Q.E.D
>> +	 *
>> +	 * We have proven that the claim is true on the assumption that
>> +	 * there is no context switch for T1 and T2. Note that the reasoning
>> +	 * of the proof uses barriers operating on the inner shareable domain,
>> +	 * which means that they will affect all CPUs, and also a context switch
>> +	 * will insert extra barriers into the code paths => the claim will
>> +	 * stand true even if we drop the assumption.
>> +	 *
>> +	 * It is also worth reasoning whether something can go wrong via
>> +	 * pud_free_pmd_page() -> __pmd_free_pte_page(), since the latter
>> +	 * will be called locklessly on this code path.
>> +	 *
>> +	 * For Case 1 (a), T2 will block until CS is finished, so we are safe.
>> +	 * For Case 1 (b) and Case 2, the PMD table will be isolated before
>> +	 * T1 can enter CS, therefore it is safe for T2 to operate on the
>> +	 * PMD table locklessly.
>> +	 */
> Although I can see that you put a lot of effort into this comment, I
> think we should just remove it. Instead, we should have a litmus test
> in the commit message and probably just some small comments here to
> explain e.g. why the mmap_read_lock() critical section is empty.
>
> I'm currently trying to put together a litmus test with James (cc'd) so
> maybe we can help you out with that part.

Thanks for the effort!

>
> In the meantime...
>
>> diff --git a/arch/arm64/mm/ptdump.c b/arch/arm64/mm/ptdump.c
>> index 421a5de806c6..65335c7ba482 100644
>> --- a/arch/arm64/mm/ptdump.c
>> +++ b/arch/arm64/mm/ptdump.c
>> @@ -283,6 +283,13 @@ void note_page_flush(struct ptdump_state *pt_st)
>>   	note_page(pt_st, 0, -1, pte_val(pte_zero));
>>   }
>>   
>> +static void arm64_ptdump_walk_pgd(struct ptdump_state *st, struct mm_struct *mm)
>> +{
>> +	static_branch_enable(&arm64_ptdump_lock_key);
>> +	ptdump_walk_pgd(st, mm, NULL);
>> +	static_branch_disable(&arm64_ptdump_lock_key);
>> +}
> What serialises the toggling of the static key here? For example, I can't
> see what prevents a kernel page-table dump running concurrently with a
> check_wx() and the key ending up in the wrong state.

Good spot. I believe static_branch_{inc, dec} should work here. The vmalloc code
only needs to know that someone is traversing the pagetables, and there is no
synchronization needed between check_wx and ptdump. And, static_branch_inc()
should have the same semantics as static_branch_enable().

>
> Either we need an additional lock or perhaps using
> static_branch_{inc,dec}() would work instead? I haven't thought too hard
> about that but it looks like we need _something_.
>
> Will