Re: [PATCH v4 0/3] KVM: Fix and account mem_attr_array reservations abandoned on ENOMEM

[Sean Christopherson]

On Tue, 15 Sep 2026 17:53:40 +0000, David Ballesteros wrote:
> Three small fixes to KVM's per-page memory attributes: release the xarray
> reservations that KVM_SET_MEMORY_ATTRIBUTES abandons when it fails partway
> through (1/3), make kvm_range_has_memory_attributes() agree with itself
> about what such a reservation means (2/3), and charge the xa_nodes to the
> caller's memcg as the code already intended (3/3).
> 
>   1/3  Release the reservations abandoned on ENOMEM.  This is a plain bug:
>        xa_reserve() materializes entries GFN-by-GFN before the store phase,
>        and on failure the loop bails without releasing what it reserved.  A
>        later clear covering them does erase them, but nothing obliges
>        userspace to issue one; absent that, the reclaim path is
>        kvm_destroy_vm().  The retained entries are not inert -- an
>        abandoned reservation is an XA_ZERO_ENTRY, which
>        kvm_range_has_memory_attributes()'s !attrs fast path counts as
>        present (raw xas_find()) while kvm_get_memory_attributes() treats
>        it as absent, so a straddling hugepage over such an entry is marked
>        mixed and KVM stops using a hugepage for a range whose attributes
>        are uniform.  xa_release() erases only entries still reserved,
>        leaving pre-existing value entries untouched.
> 
> [...]

Applied patch 3, with a heavily modified changelog, to kvm-x86 fixes.  For the
reservation behavior, I went with Zeng Chi's fix to have KVM treat ZERO values
as "no attributes".  Having dangling reservations is a-ok, the memcg accounting
really needs to do the right thing there.

In the future, please don't have AI directly write changelogs.  It's fine to
let AI generate a rough draft, for me at least, AI tends to be far too verbose
and uses terminology that isn't common in Linux/upstream.  In other words, AI
tends to write changelogs (and bug reports) that require far too much effort
to understand.

I apologize in advance if you wrote the changelogs, i.e. if I am falsely
accusing you of being a robot.  If AI didn't write the changelogs, well, you
do one heck of a job of imitating some of my newfound "friends" :-)

Gripes about AI aside, than you very much for the fixes!

[1/3] KVM: Release memory-attribute reservations abandoned on ENOMEM
      [SKIP]
[2/3] KVM: Make kvm_range_has_memory_attributes() consistent about reservations
      [SKIP]
[3/3] KVM: Account mem_attr_array nodes to the caller's memcg
      https://github.com/kvm-x86/linux/commit/382e5d514b6f

--
https://github.com/kvm-x86/linux/tree/next