From cf7e7a3503df0b71afd68ee84e9a09d4514cc2dd Mon Sep 17 00:00:00 2001 From: Lorenzo Stoakes Date: Thu, 29 May 2025 18:15:47 +0100 Subject: mm: prevent KSM from breaking VMA merging for new VMAs If a user wishes to enable KSM mergeability for an entire process and all fork/exec'd processes that come after it, they use the prctl() PR_SET_MEMORY_MERGE operation. This defaults all newly mapped VMAs to have the VM_MERGEABLE VMA flag set (in order to indicate they are KSM mergeable), as well as setting this flag for all existing VMAs and propagating this across fork/exec. However it also breaks VMA merging for new VMAs, both in the process and all forked (and fork/exec'd) child processes. This is because when a new mapping is proposed, the flags specified will never have VM_MERGEABLE set. However all adjacent VMAs will already have VM_MERGEABLE set, rendering VMAs unmergeable by default. To work around this, we try to set the VM_MERGEABLE flag prior to attempting a merge. In the case of brk() this can always be done. However on mmap() things are more complicated - while KSM is not supported for MAP_SHARED file-backed mappings, it is supported for MAP_PRIVATE file-backed mappings. These mappings may have deprecated .mmap() callbacks specified which could, in theory, adjust flags and thus KSM eligibility. So we check to determine whether this is possible. If not, we set VM_MERGEABLE prior to the merge attempt on mmap(), otherwise we retain the previous behaviour. This fixes VMA merging for all new anonymous mappings, which covers the majority of real-world cases, so we should see a significant improvement in VMA mergeability. For MAP_PRIVATE file-backed mappings, those which implement the .mmap_prepare() hook and shmem are both known to be safe, so we allow these, disallowing all other cases. Also add stubs for newly introduced function invocations to VMA userland testing. [lorenzo.stoakes@oracle.com: correctly invoke late KSM check after mmap hook] Link: https://lkml.kernel.org/r/5861f8f6-cf5a-4d82-a062-139fb3f9cddb@lucifer.local Link: https://lkml.kernel.org/r/3ba660af716d87a18ca5b4e635f2101edeb56340.1748537921.git.lorenzo.stoakes@oracle.com Fixes: d7597f59d1d3 ("mm: add new api to enable ksm per process") # please no backport! Signed-off-by: Lorenzo Stoakes Reviewed-by: Chengming Zhou Acked-by: David Hildenbrand Reviewed-by: Liam R. Howlett Reviewed-by: Vlastimil Babka Reviewed-by: Xu Xin Cc: Al Viro Cc: Christian Brauner Cc: Jan Kara Cc: Jann Horn Cc: Stefan Roesch Signed-off-by: Andrew Morton --- include/linux/ksm.h | 8 +++++--- 1 file changed, 5 insertions(+), 3 deletions(-) (limited to 'include/linux') diff --git a/include/linux/ksm.h b/include/linux/ksm.h index d73095b5cd96..51787f0b0208 100644 --- a/include/linux/ksm.h +++ b/include/linux/ksm.h @@ -17,8 +17,8 @@ #ifdef CONFIG_KSM int ksm_madvise(struct vm_area_struct *vma, unsigned long start, unsigned long end, int advice, unsigned long *vm_flags); - -void ksm_add_vma(struct vm_area_struct *vma); +vm_flags_t ksm_vma_flags(const struct mm_struct *mm, const struct file *file, + vm_flags_t vm_flags); int ksm_enable_merge_any(struct mm_struct *mm); int ksm_disable_merge_any(struct mm_struct *mm); int ksm_disable(struct mm_struct *mm); @@ -97,8 +97,10 @@ bool ksm_process_mergeable(struct mm_struct *mm); #else /* !CONFIG_KSM */ -static inline void ksm_add_vma(struct vm_area_struct *vma) +static inline vm_flags_t ksm_vma_flags(const struct mm_struct *mm, + const struct file *file, vm_flags_t vm_flags) { + return vm_flags; } static inline int ksm_disable(struct mm_struct *mm) -- cgit v1.2.3