Re: [PATCH v5 0/4] mm/huge_memory: copy PMDs at fork like the PTE path does

[Andrew Morton <akpm@linux-foundation.org>]

On Thu, 5 Nov 2026 21:18:19 -0800 Andrew Morton <akpm@linux-foundation.org> wrote:

> I'm attempting to runtime test this on local LLM-driven live kernel
> test setup, but it keeps on kicking me out and calling me a
> cybercriminal.  Sigh.  Giving up

Success.

tl;dr: all seems good.  The expected performance gains and losses were
observed.  Can't performance-test powerpc because the unpatched kernels
crashed due to the bug the series fixes.

: I tested the four v5 patches and the selftest fix on matched x86-64
: kernels. Both shmem fork tests passed with the series and skipped on
: the pre-series kernel, as expected. They checked inherited PMDs,
: private and shared writes, parent mappings and ShmemPmdMapped after
: child exit.
: 
: The special-PMD race was reproduced with a virtual QXL device mapped
: through VFIO. Revocation before fork produced SIGBUS on both kernels.
: When revocation completed during a controlled pause in fork, the child
: accessed the revoked mapping before the series and received SIGBUS with
: the series. Both pauses were reached without timing out. No BUG,
: WARNING, KASAN report or oops appeared in these x86-64 runs.
: 
: I also built matched PowerPC kernels and tested a POWER9/pseries
: hash-MMU guest under software emulation: two vCPUs, 2 GiB RAM, 64 KiB
: base pages and 16 MiB PMDs. A trace confirmed order-8 VFIO PMD
: insertion from a 64 MiB virtual PCI shared-memory BAR. On the
: pre-series kernel, the child's first access after fork caused a NULL
: pointer dereference in __hash_page_thp() and a soft lockup; that guest
: required a bounded timeout. With the series, the child read and exited
: normally. Revocation before fork gave the child SIGBUS with the series.
: This is one comparison, not a PowerPC stress test.
: 
: I measured fork costs with a 512 MiB private shmem mapping, including
: 510 MiB actually mapped with PMDs. Median times from 100 forks per
: condition on each x86-64 kernel, following five warmups:
: 
:                          Before    With series
:   fork only                49 us       63 us
:   fork through child exit,
:     no child reads        132 us      202 us
:   fork through child exit,
:     one read per PMD      637 us      326 us
: 
: This workload pays more at fork and takes about half as long end-to-end
: when the child reads every PMD. These are single-boot measurements, not
: a broader performance characterization. The timing kernels had KASAN,
: lockdep and DEBUG_VM disabled; the correctness kernels had them enabled.
: The x86-64 KVM guests had 8 GiB, four vCPUs (one virtual socket, four
: single-threaded virtual cores) and one virtual NUMA node. The host has
: 64 physical cores and 128 SMT threads on one physical NUMA node;
: individual vCPUs were not pinned.
: 
: I attempted a PowerPC VFIO fork-only timing comparison. The pre-series
: kernel faulted on the first child exit, even without a child read, and
: the guest was stopped after a 90-second timeout. The patched kernel
: completed 100 fork/exit operations, averaging 2.77 ms under software
: emulation with tracing enabled. There is no matched PowerPC performance
: comparison.
: 
: Tested-by: Andrew Morton <akpm@linux-foundation.org>