On 13.11.23 10:33, Xu Yu wrote:
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@xxxxxxxxxx>
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. :(
I think the simplest two cases to handle that we should implement:
no subpages are PageAnonExclusive (PTEs must be R/O) -> we can collapse
into a R/O PMD without further action.
all subpages are PageAnonExclusive (PTEs may be either R/O or R/W) ->
clear PageAnonExclusive on all tail pages but the first (head) page and
collapse to a R/W PMD with VM_WRITE or a R/O PMD without VM_WRITE.
We must check PageAnonExclusive with the page table lock held, and
recheck in case we dropped it (IOW, what existing collapse code already
does).
softdirty tracking and uffd-wp checks should likely be the same as for
existing collapse handling.
--
Cheers,
David / dhildenb
