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

From: Ryan Roberts
Date: Wed Sep 04 2024 - 07:36:33 EST


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@xxxxxxx/
> [2]: https://lore.kernel.org/all/1cfae0c0-96a2-4308-9c62-f7a640520242@xxxxxxx/
>
> 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.

Thanks,
Ryan