Re: [PATCH 1/1] mm/khugepaged: map anonymous pte-mapped THPs by pmds

[David Hildenbrand <david@redhat.com>]

On 13.11.23 10:05, Xu Yu wrote:
> In the anonymous collapse path, khugepaged collapses pte-mapped
> hugepages by allocating and copying to a new hugepage, which is
> suboptimally.
> 
> In fact, we only need to update the mapping page tables for anonymous
> pte-mapped THPs, in the same way as file/shmem-backed pte-mapped THPs,
> as shown in commit 58ac9a8993a1 ("mm/khugepaged: attempt to map
> file/shmem-backed pte-mapped THPs by pmds").

Somewhere along that patch set discussion, there was a discussion about
how to deal with PageAnonExclusive properly.

Quite a lot of information is documented in:

commit dee2ad120571f38433211098cd6b95a59bdfc8c7
Author: David Hildenbrand <david@redhat.com>
Date:   Wed Jan 4 15:49:05 2023 +0100

     selftests/vm: cow: add COW tests for collapsing of PTE-mapped anon THP
     
     Currently, anonymous PTE-mapped THPs cannot be collapsed in-place:
     collapsing (e.g., via MADV_COLLAPSE) implies allocating a fresh THP and
     mapping that new THP via a PMD: as it's a fresh anon THP, it will get the
     exclusive flag set on the head page and everybody is happy.
...



I purely speculate that you didn't consider it and got it wrong, sorry :P

Fortunately, I wrote a test case for that where it's documented (above)

tools/testing/selftests/mm/cow.c tests:

# [RUN] Basic COW after fork() when collapsing before fork()
ok 169 No leak from parent into child
# [RUN] Basic COW after fork() when collapsing after fork() (fully shared)
ok 170 # SKIP MADV_COLLAPSE failed: Invalid argument
# [RUN] Basic COW after fork() when collapsing after fork() (lower shared)
ok 171 No leak from parent into child
# [RUN] Basic COW after fork() when collapsing after fork() (upper shared)
ok 172 No leak from parent into child


If your implementation supports MADV_COLLAPSE, it might reveal any problems.

-- 
Cheers,

David / dhildenb