Re: [PATCH v3 17/40] mm/vma: add and use vma_[flags]_is_fixed_mapping
From: "Zi Yan" <ziy@nvidia.com>
Date: 2026-09-26 02:27:32
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bpf, fuse-devel, kvm, kvm-riscv, kvmarm, linux-arch, linux-doc, linux-fsdevel, linux-mm, linux-perf-users, linux-rdma, linux-s390, linux-scsi, linux-sound, linux-trace-kernel, linux-usb, linuxppc-dev, lkml, selinux, sparclinux
On Thu Sep 17, 2026 at 12:22 PM EDT, Lorenzo Stoakes (ARM) wrote:
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. Also update the userland VMA tests to reflect the change. 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 ++--- tools/testing/vma/include/dup.h | 16 +++++++++++++++- 4 files changed, 59 insertions(+), 12 deletions(-)
<snip>
quoted hunk ↗ jump to hunk
diff --git a/mm/huge_memory.c b/mm/huge_memory.c index 4cd917f77f3f..4d0acd9a1099 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))
I wonder if a function alias would improve the code readability. Basically, #define vma_no_khugepaged vma_can_merge or just make vma_no_khugepaged static inline. And move the comment to the function. Regardless, this patch makes sense to me. Reviewed-by: Zi Yan <ziy@nvidia.com> -- Best Regards, Yan, Zi