[0/2] Do not shatter hugezeropage on wp-fault

Message

Dev Jain Aug. 30, 2024, 8:41 a.m. UTC

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/[e-mail]/
[2]: https://lore.kernel.org/all/[e-mail]/

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(-)