[PATCH 0/1] Clarify huge_pte_offset() semantics

[Punit Agrawal]

Hi,

The following patch is an attempt to make huge_pte_offset() consistent
when dealing with different levels of the page table and document the
expected semantics. Previously posting can be found at [0].

Changelog

RFC - v1
* Merge Patch 1 and 2 - preserve bisectability
* Drop RFC tag

Original cover letter follows...

The generic implementation of huge_pte_offset() has inconsistent
behaviour when looking up hugepage PUDs vs PMDs entries that are not
present (returning NULL vs pte_t*).

Similarly, it returns NULL when encountering swap entries although all
the callers have special checks to properly deal with swap entries.

Without clear semantics, it is difficult to determine if a change
breaks huge_pte_offset() without going through all the scenarios where
it is used.

I faced this recently when updating the arm64 implementation of
huge_pte_offset() to handle swap entries (related to enabling poisoned
memeory)[1]. And will come across again when I update it for
contiguous hugepage support now that core changes have been merged.

To address these issues, this following patch -

* makes huge_pte_offset() consistent between PUD and PMDs
* and, documents the expected behaviour of huge_pte_offset()

All feedback welcome.

Thanks,
Punit

[0] https://lkml.org/lkml/2017/7/24/514
[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=f02ab08afbe76ee7b0b2a34a9970e7dd200d8b01

Punit Agrawal (1):
  mm/hugetlb: Make huge_pte_offset() consistent and document behaviour

 mm/hugetlb.c | 22 +++++++++++++++++++---
 1 file changed, 19 insertions(+), 3 deletions(-)

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2.11.0