Re: [PATCH v2 0/3] mm/hmm: Add mmap lock-drop support for userfaultfd-backed mappings

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

On Wed, 13 May 2026 02:40:11 +0000 Stanislav Kinsburskii <skinsburskii@linux.microsoft.com> wrote:

> This series extends the HMM framework to support userfaultfd-backed memory
> by allowing the mmap read lock to be dropped during hmm_range_fault().
> 
> Some page fault handlers — most notably userfaultfd — require the mmap lock
> to be released so that userspace can resolve the fault. The current HMM
> interface never sets FAULT_FLAG_ALLOW_RETRY, making it impossible to fault
> in pages from userfaultfd-registered regions.
> 
> This series follows the established int *locked pattern from
> get_user_pages_remote() in mm/gup.c. A new entry point,
> hmm_range_fault_unlockable(), accepts an int *locked parameter. When the
> mmap lock is dropped during fault resolution (VM_FAULT_RETRY or
> VM_FAULT_COMPLETED), the function returns 0 with *locked = 0, signalling
> the caller to restart its walk. The existing hmm_range_fault() is
> refactored into a thin wrapper that passes NULL, preserving current
> behavior for all existing callers.
> 
> Faulting hugetlb pages on the unlockable path is not supported because
> walk_hugetlb_range() unconditionally holds and releases
> hugetlb_vma_lock_read across the callback; if the mmap lock is dropped
> inside the callback, the VMA may be freed before the walk framework's
> unlock. Hugetlb pages already present in page tables are handled normally.
> Possible approaches to lift this limitation are documented in
> Documentation/mm/hmm.rst.

Thanks.  AI review asked some questions:
	https://sashiko.dev/#/patchset/177863991557.82528.15288076059759579141.stgit@skinsburskii-cloud-desktop.internal.cloudapp.net

I'd ignore the fist one: don't write buggy fault handlers!