Re: [PATCH mm-unstable] mm/khugepaged: fix collapse_pte_mapped_thp() versus uffd

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On 22.08.23 16:39, Jann Horn wrote:
On Tue, Aug 22, 2023 at 4:51 AM Hugh Dickins <hughd@xxxxxxxxxx> wrote:
On Mon, 21 Aug 2023, Jann Horn wrote:
On Mon, Aug 21, 2023 at 9:51 PM Hugh Dickins <hughd@xxxxxxxxxx> wrote:
Just for this case, take the pmd_lock() two steps earlier: not because
it gives any protection against this case itself, but because ptlock
nests inside it, and it's the dropping of ptlock which let the bug in.
In other cases, continue to minimize the pmd_lock() hold time.

Special-casing userfaultfd like this makes me a bit uncomfortable; but
I also can't find anything other than userfaultfd that would insert
pages into regions that are khugepaged-compatible, so I guess this

I'm as sure as I can be that it's solely because userfaultfd breaks
the usual rules here (and in fairness, IIRC Andrea did ask my permission
before making it behave that way on shmem, COWing without a source page).

Perhaps something else will want that same behaviour in future (it's
tempting, but difficult to guarantee correctness); for now, it is just
userfaultfd (but by saying "_armed" rather than "_missing", I'm half-
expecting uffd to add more such exceptional modes in future).

Hm, yeah, sounds okay. (I guess we'd also run into this if we ever
wanted to make it possible to reliably install PTE markers with
madvise() or something like that, which might be nice for allowing
userspace to create guard pages without unnecessary extra VMAs...)

I'm working on something similar that goes a bit further than just guard pages. It also installs PTE markers into page tables, inside existing large VMAs.

Initially, I'll only tackle anon VMAs, though.


David / dhildenb

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