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. [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 | 7 ++ mm/huge_memory.c | 171 +++++++++++++++++++++++++++++----------- mm/memory.c | 5 +- 3 files changed, 136 insertions(+), 47 deletions(-) -- 2.30.2
