Re: [PATCH v2 0/2] Do not shatter hugezeropage on wp-fault

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

On 9/4/24 17:06, Ryan Roberts wrote:
> Hi Dev,
>
> On 04/09/2024 11:09, Dev Jain wrote:
>> It was observed at [1] and [2] that the current kernel behaviour of
>> shattering a hugezeropage is inconsistent and suboptimal. For a VMA with
>> a THP allowable order, when we write-fault on it, the kernel installs a
>> PMD-mapped THP. On the other hand, if we first get a read fault, we get
>> a PMD pointing to the hugezeropage; subsequent write will trigger a
>> write-protection fault, shattering the hugezeropage into one writable
>> page, and all the other PTEs write-protected. The conclusion being, as
>> compared to the case of a single write-fault, applications have to suffer
>> 512 extra page faults if they were to use the VMA as such, plus we get
>> the overhead of khugepaged trying to replace that area with a THP anyway.
>>
>> Instead, replace the hugezeropage with a THP on wp-fault.
>>
>> v1->v2:
>>   - Wrap do_huge_zero_wp_pmd_locked() around lock and unlock
>>   - Call thp_fault_alloc() before do_huge_zero_wp_pmd_locked() to avoid
>>   - calling sleeping function from spinlock context
>>
>> [1]: https://lore.kernel.org/all/3743d7e1-0b79-4eaf-82d5-d1ca29fe347d@arm.com/
>> [2]: https://lore.kernel.org/all/1cfae0c0-96a2-4308-9c62-f7a640520242@arm.com/
>>
>> Dev Jain (2):
>>    mm: Abstract THP allocation
>>    mm: Allocate THP on hugezeropage wp-fault
>>
>>   include/linux/huge_mm.h |   6 ++
>>   mm/huge_memory.c        | 171 +++++++++++++++++++++++++++++-----------
>>   mm/memory.c             |   5 +-
>>   3 files changed, 136 insertions(+), 46 deletions(-)
>>
> What is the base for this? It doesn't apply on top of mm-unstable.

Sorry, forgot to mention, it applies on v6.11-rc5.

>
> Thanks,
> Ryan
>