Re: [PATCH 2/2] arm64/mm: Improve comment in contpte_ptep_get_lockless()

[Ryan Roberts <ryan.roberts@arm.com>]

On 26/02/2024 12:30, David Hildenbrand wrote:
> On 26.02.24 13:03, Ryan Roberts wrote:
>> Make clear the atmicity/consistency requirements of the API and how we
>> achieve them.
>>
>> Link: https://lore.kernel.org/linux-mm/Zc-Tqqfksho3BHmU@arm.com/
>> Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
>> ---
>>   arch/arm64/mm/contpte.c | 24 ++++++++++++++----------
>>   1 file changed, 14 insertions(+), 10 deletions(-)
>>
>> diff --git a/arch/arm64/mm/contpte.c b/arch/arm64/mm/contpte.c
>> index be0a226c4ff9..1b64b4c3f8bf 100644
>> --- a/arch/arm64/mm/contpte.c
>> +++ b/arch/arm64/mm/contpte.c
>> @@ -183,16 +183,20 @@ EXPORT_SYMBOL_GPL(contpte_ptep_get);
>>   pte_t contpte_ptep_get_lockless(pte_t *orig_ptep)
>>   {
>>       /*
>> -     * Gather access/dirty bits, which may be populated in any of the ptes
>> -     * of the contig range. We may not be holding the PTL, so any contiguous
>> -     * range may be unfolded/modified/refolded under our feet. Therefore we
>> -     * ensure we read a _consistent_ contpte range by checking that all ptes
>> -     * in the range are valid and have CONT_PTE set, that all pfns are
>> -     * contiguous and that all pgprots are the same (ignoring access/dirty).
>> -     * If we find a pte that is not consistent, then we must be racing with
>> -     * an update so start again. If the target pte does not have CONT_PTE
>> -     * set then that is considered consistent on its own because it is not
>> -     * part of a contpte range.
>> +     * The ptep_get_lockless() API requires us to read and return *orig_ptep
>> +     * so that it is self-consistent, without the PTL held, so we may be
>> +     * racing with other threads modifying the pte. Usually a READ_ONCE()
>> +     * would suffice, but for the contpte case, we also need to gather the
>> +     * access and dirty bits from across all ptes in the contiguous block,
>> +     * and we can't read all of those neighbouring ptes atomically, so any
>> +     * contiguous range may be unfolded/modified/refolded under our feet.
>> +     * Therefore we ensure we read a _consistent_ contpte range by checking
>> +     * that all ptes in the range are valid and have CONT_PTE set, that all
>> +     * pfns are contiguous and that all pgprots are the same (ignoring
>> +     * access/dirty). If we find a pte that is not consistent, then we must
>> +     * be racing with an update so start again. If the target pte does not
>> +     * have CONT_PTE set then that is considered consistent on its own
>> +     * because it is not part of a contpte range.
>>        */
>>         pgprot_t orig_prot;
> 
> Reviewed-by: David Hildenbrand <david@redhat.com>

Thanks!

> 
> In an ideal world, we'd really not rely on any accessed/dirty on the lockless
> path and remove contpte_ptep_get_lockless() completely :)

Not sure if you saw my RFC to do exactly that? (well, it doesn't actually remove
[contpte_]ptep_get_lockless() but it does remove all the callers). If you have
any feedback, we could get this moving...

https://lore.kernel.org/linux-mm/20240215121756.2734131-1-ryan.roberts@arm.com/

Thanks,
Ryan