linux-kernel - [PATCH v2 0/2] Do not shatter hugezeropage on wp-fault

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Message-Id: <20240904100923.290042-1-dev.jain@arm.com>
Date: Wed,  4 Sep 2024 15:39:21 +0530
From: Dev Jain <dev.jain@....com>
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	Dev Jain <dev.jain@....com>
Subject: [PATCH v2 0/2] Do not shatter hugezeropage on wp-fault

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(-)

-- 
2.30.2