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

[Xu Yu <xuyu@linux.alibaba.com>]

On 11/13/23 5:26 PM, David Hildenbrand wrote:
> 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

As you said, I did not consider it. Actually, I didn't even know that. :(

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

Thanks very much! I will dig into it.

> 
> 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.
> 
-- 
Thanks,
Yu