[PATCH 16/39] mm/vma: add and use vma_[flags]_is_fixed_mapping
From: "Lorenzo Stoakes (ARM)" <ljs@kernel.org>
Date: 2026-09-08 20:10:04
Also in:
bpf, dri-devel, fuse-devel, kvm, kvm-riscv, kvmarm, linux-arch, linux-doc, linux-fbdev, linux-fsdevel, linux-mm, linux-perf-users, linux-rdma, linux-s390, linux-scsi, linux-sound, linux-usb, linuxppc-dev, lkml, selinux, sparclinux
Subsystem:
memory management, memory management - core, memory management - thp (transparent huge page), memory mapping, the rest · Maintainers:
Andrew Morton, David Hildenbrand, Liam R. Howlett, Lorenzo Stoakes, Linus Torvalds
This determines whether a VMA cannot be expanded or merged because what they mapped was determined to be a set size at mmap time. This typically refers to kernel-owned mappings, however VMA_DONTEXPAND_BIT is not reliably set alongside VMA_PFNMAP_BIT or VMA_MIXEDMAP_BIT, so we must explicitly test for this for now. We also explicitly test for VMA_PFNMAP_BIT as VMA_DONTEXPAND_BIT may not be set for VMA_PFNMAP_BIT's despite the one implying the other. Use this predicate in vma_flags_can_merge() and in check_prep_vma() in the mremap logic testing to see if mremap() can expand the VMA. The criteria for khugepaged and MADV_COLLAPSE eligibility in __thp_vma_allowable_orders() are precisely those for mergeability, so use vma_can_merge() there (with an expanded comment). This obviates the need for the VM_NO_KHUGEPAGED mask, so remove it. Hugetlb VMAs remain excluded from khugepaged as hugetlbfs always sets VMA_DONTEXPAND_BIT. No functional change intended. Signed-off-by: Lorenzo Stoakes (ARM) <ljs@kernel.org> --- include/linux/mm.h | 39 +++++++++++++++++++++++++++++++++++---- mm/huge_memory.c | 11 +++++++---- mm/mremap.c | 5 ++--- 3 files changed, 44 insertions(+), 11 deletions(-)
diff --git a/include/linux/mm.h b/include/linux/mm.h
index 45c59474c853..bca955941212 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h@@ -600,9 +600,6 @@ enum { #define VMA_REMAP_FLAGS mk_vma_flags(VMA_IO_BIT, VMA_PFNMAP_BIT, \ VMA_DONTEXPAND_BIT, VMA_DONTDUMP_BIT) -/* This mask prevents VMA from being scanned with khugepaged */ -#define VM_NO_KHUGEPAGED (VM_SPECIAL | VM_HUGETLB) - /* This mask defines which mm->def_flags a process can inherit its parent */ #define VM_INIT_DEF_MASK VM_NOHUGEPAGE
@@ -1650,6 +1647,40 @@ static inline bool vma_is_kernel_owned(const struct vm_area_struct *vma) return vma_flags_is_kernel_owned(&vma->flags); } +/** + * vma_flags_is_fixed_mapping() - Do the specified VMA flags indicate that this + * is a fixed mapping that cannot be expanded or merged? + * @flags: The VMA flags to test. + * + * Fixed mappings are those whose size is set at the point of mmap (for + * instance, a kernel-owned mapping of a fixed range of memory), and thus + * cannot be expanded or merged. + * + * Returns: true if the flags indicate a fixed mapping. + */ +static inline bool vma_flags_is_fixed_mapping(const vma_flags_t *flags) +{ + /* + * VMA_PFNMAP_BIT should imply VMA_DONTEXPAND_BIT, but some callers set + * only the former. + */ + return vma_flags_test_any(flags, VMA_PFNMAP_BIT, VMA_DONTEXPAND_BIT); +} + +/** + * vma_is_fixed_mapping() - Is this VMA a fixed mapping that cannot be + * expanded or merged? + * @vma: The VMA to test. + * + * See vma_flags_is_fixed_mapping() for a description of this property. + * + * Returns: true if the VMA maps a fixed mapping. + */ +static inline bool vma_is_fixed_mapping(const struct vm_area_struct *vma) +{ + return vma_flags_is_fixed_mapping(&vma->flags); +} + /** * vma_flags_can_merge() - Do the specified VMA flags permit the VMA to be * merged with another?
@@ -1671,7 +1702,7 @@ static inline bool vma_flags_can_merge(const vma_flags_t *flags) if (vma_flags_is_kernel_owned(flags)) return false; /* VMA explicitly marked as being unmergeable. */ - if (vma_flags_test(flags, VMA_DONTEXPAND_BIT)) + if (vma_flags_is_fixed_mapping(flags)) return false; return true;
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index dd66c6ad5af1..befffadd978e 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c@@ -212,11 +212,14 @@ unsigned long __thp_vma_allowable_orders(struct vm_area_struct *vma, return in_pf ? orders : 0; /* - * khugepaged special VMA and hugetlb VMA. - * Must be checked after dax since some dax mappings may have - * VM_MIXEDMAP set. + * khugepaged moves data from VMAs once collapsed, after they have been + * faulted in, relying on refaulting for file-backed memory. + * + * Kernel-owned mappings cannot be reliably reconstructed from page + * faults, and fixed mappings (including hugetlb) may not be marked as + * kernel-owned - precisely the mappings which cannot be merged. */ - if (!in_pf && !smaps && (vm_flags & VM_NO_KHUGEPAGED)) + if (!in_pf && !smaps && !vma_can_merge(vma)) return 0; /*
diff --git a/mm/mremap.c b/mm/mremap.c
index 7c368440fafe..ed19b47c2caf 100644
--- a/mm/mremap.c
+++ b/mm/mremap.c@@ -1788,8 +1788,7 @@ static int check_prep_vma(struct vma_remap_struct *vrm) return -EINVAL; } - if ((vrm->flags & MREMAP_DONTUNMAP) && - vma_test_any(vma, VMA_DONTEXPAND_BIT, VMA_PFNMAP_BIT)) + if ((vrm->flags & MREMAP_DONTUNMAP) && vma_is_fixed_mapping(vma)) return -EINVAL; /*
@@ -1827,7 +1826,7 @@ static int check_prep_vma(struct vma_remap_struct *vrm) if (pgoff + (new_len >> PAGE_SHIFT) < pgoff) return -EINVAL; - if (vma_test_any(vma, VMA_DONTEXPAND_BIT, VMA_PFNMAP_BIT)) + if (vma_is_fixed_mapping(vma)) return -EFAULT; if (!mlock_future_ok(mm, vma_test(vma, VMA_LOCKED_BIT), vrm->delta))
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2.55.0