Re: [PATCH v3 15/40] mm/vma: add vma[_flags]_is_kernel_owned() predicates
From: "David Hildenbrand (Arm)" <david@kernel.org>
Date: 2026-10-01 13:18:38
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On 9/17/26 18:22, Lorenzo Stoakes (ARM) wrote:
Rather than referring to VMA flags with uncertain meaning, add a new predicate that explicitly describes what possession of the VMA_PFNMAP_BIT or VMA_MIXEDMAP_BIT flags mean, and then refer to that function for determining VMA mergeability. Either flag means the contents of the mapping are owned by the kernel, usually a driver, rather than by the core mm: the memory may be MMIO, kernel-allocated pages or even ordinary pages the driver maps itself, but the core must not populate, reclaim, migrate, copy-on-write or merge the range on its own initiative. We initially also include VMA_IO_BIT here, as by implication, these must be kernel-owned. (mlock() also sets VMA_IO_BIT transiently on ordinary VMAs while locking them, which is addressed later in this series.) However the intent is to in future remove this, as no mapping should be marked as an I/O mapping without also being marked with VMA_PFNMAP_BIT. This forms the basis of further work intended to improve how we express VMA properties such as this. Also update the VMA userland tests to reflect the change. No functional change intended.
Of course I have to bitch about the naming :) Intuitively: kernel owned vs ... user owned? No, it's kernel owned vs core-mm owned. Which implies core-mm is not part of the kernel? Yes, this is confusing. ;) I assume you're coming from "map_kernel_pages*", but that's rather "kernel memory" and not "kernel owned". Usually we say "driver owned" when not talking about pagecache/anon. Or user vs. kernel memory. So is it really all about "is this (excluding CoW) no ordinary user memory that we would track through the rmap" ?
quoted hunk ↗ jump to hunk
Signed-off-by: Lorenzo Stoakes (ARM) <ljs@kernel.org> --- include/linux/mm.h | 56 ++++++++++++++++++++++++++++++++++++++++- tools/testing/vma/include/dup.h | 29 ++++++++++++++++++++- 2 files changed, 83 insertions(+), 2 deletions(-)diff --git a/include/linux/mm.h b/include/linux/mm.h index 2a92193ac6a5..cab29d6e15c1 100644 --- a/include/linux/mm.h +++ b/include/linux/mm.h@@ -1612,6 +1612,44 @@ static inline bool vma_is_shared_maywrite(const struct vm_area_struct *vma) return is_shared_maywrite(&vma->flags); } +/** + * vma_flags_is_kernel_owned() - Do the specified VMA flags indicate that the + * contents of the VMA are owned by the kernel rather than the core mm? + * @flags: The VMA flags to test. + * + * A kernel-owned mapping is one whose contents are established and controlled + * by the kernel, typically a driver, rather than by the core mm's fault and + * rmap machinery. + * + * The mapping may be memory-mapped I/O, kernel-allocated pages or ordinary + * pages the owner has chosen to map itself (shmem via a PFN map, for instance). + * + * In all cases the core mm must not populate, reclaim, migrate, copy-on-write + * or merge it of its own accord. + * + * Pages mapped this way are not necessarily reference counted or map counted. + * + * Returns: true if the flags indicate a kernel-owned mapping. + */ +static inline bool vma_flags_is_kernel_owned(const vma_flags_t *flags) +{ + return vma_flags_test_any(flags, VMA_PFNMAP_BIT, VMA_MIXEDMAP_BIT, + VMA_IO_BIT);
I thought we have cases where we drivers insert pages and neither set VMA_PFNMAP_BIT nor VMA_MIXEDMAP_BIT. I assume vmf_insert_page_mkwrite() is fine because it is DAX doing it (should we limit this interface to DAX?). -- Cheers, David