From cf7e7a3503df0b71afd68ee84e9a09d4514cc2dd Mon Sep 17 00:00:00 2001 From: Lorenzo Stoakes Date: Thu, 29 May 2025 18:15:47 +0100 Subject: [PATCH] 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 +++-- mm/ksm.c | 18 +++++++---- mm/vma.c | 51 ++++++++++++++++++++++++++++++-- tools/testing/vma/vma_internal.h | 11 +++++++ 4 files changed, 77 insertions(+), 11 deletions(-) diff --git a/include/linux/ksm.h b/include/linux/ksm.h index d73095b5cd96e..51787f0b0208f 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) diff --git a/mm/ksm.c b/mm/ksm.c index d0c763abd4992..18b3690bb69ad 100644 --- a/mm/ksm.c +++ b/mm/ksm.c @@ -2731,16 +2731,22 @@ static int __ksm_del_vma(struct vm_area_struct *vma) return 0; } /** - * ksm_add_vma - Mark vma as mergeable if compatible + * ksm_vma_flags - Update VMA flags to mark as mergeable if compatible * - * @vma: Pointer to vma + * @mm: Proposed VMA's mm_struct + * @file: Proposed VMA's file-backed mapping, if any. + * @vm_flags: Proposed VMA"s flags. + * + * Returns: @vm_flags possibly updated to mark mergeable. */ -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) { - struct mm_struct *mm = vma->vm_mm; + if (test_bit(MMF_VM_MERGE_ANY, &mm->flags) && + __ksm_should_add_vma(file, vm_flags)) + vm_flags |= VM_MERGEABLE; - if (test_bit(MMF_VM_MERGE_ANY, &mm->flags)) - __ksm_add_vma(vma); + return vm_flags; } static void ksm_add_vmas(struct mm_struct *mm) diff --git a/mm/vma.c b/mm/vma.c index fef67a66a0959..079540ebfb72e 100644 --- a/mm/vma.c +++ b/mm/vma.c @@ -32,6 +32,9 @@ struct mmap_state { struct vma_munmap_struct vms; struct ma_state mas_detach; struct maple_tree mt_detach; + + /* Determine if we can check KSM flags early in mmap() logic. */ + bool check_ksm_early; }; #define MMAP_STATE(name, mm_, vmi_, addr_, len_, pgoff_, flags_, file_) \ @@ -2320,6 +2323,11 @@ static void vms_abort_munmap_vmas(struct vma_munmap_struct *vms, vms_complete_munmap_vmas(vms, mas_detach); } +static void update_ksm_flags(struct mmap_state *map) +{ + map->flags = ksm_vma_flags(map->mm, map->file, map->flags); +} + /* * __mmap_prepare() - Prepare to gather any overlapping VMAs that need to be * unmapped once the map operation is completed, check limits, account mapping @@ -2424,6 +2432,7 @@ static int __mmap_new_file_vma(struct mmap_state *map, !(map->flags & VM_MAYWRITE) && (vma->vm_flags & VM_MAYWRITE)); + map->file = vma->vm_file; map->flags = vma->vm_flags; return 0; @@ -2473,6 +2482,11 @@ static int __mmap_new_vma(struct mmap_state *map, struct vm_area_struct **vmap) if (error) goto free_iter_vma; + if (!map->check_ksm_early) { + update_ksm_flags(map); + vm_flags_init(vma, map->flags); + } + #ifdef CONFIG_SPARC64 /* TODO: Fix SPARC ADI! */ WARN_ON_ONCE(!arch_validate_flags(map->flags)); @@ -2490,7 +2504,6 @@ static int __mmap_new_vma(struct mmap_state *map, struct vm_area_struct **vmap) */ if (!vma_is_anonymous(vma)) khugepaged_enter_vma(vma, map->flags); - ksm_add_vma(vma); *vmap = vma; return 0; @@ -2593,6 +2606,35 @@ static void set_vma_user_defined_fields(struct vm_area_struct *vma, vma->vm_private_data = map->vm_private_data; } +/* + * Are we guaranteed no driver can change state such as to preclude KSM merging? + * If so, let's set the KSM mergeable flag early so we don't break VMA merging. + */ +static bool can_set_ksm_flags_early(struct mmap_state *map) +{ + struct file *file = map->file; + + /* Anonymous mappings have no driver which can change them. */ + if (!file) + return true; + + /* + * If .mmap_prepare() is specified, then the driver will have already + * manipulated state prior to updating KSM flags. So no need to worry + * about mmap callbacks modifying VMA flags after the KSM flag has been + * updated here, which could otherwise affect KSM eligibility. + */ + if (file->f_op->mmap_prepare) + return true; + + /* shmem is safe. */ + if (shmem_file(file)) + return true; + + /* Any other .mmap callback is not safe. */ + return false; +} + static unsigned long __mmap_region(struct file *file, unsigned long addr, unsigned long len, vm_flags_t vm_flags, unsigned long pgoff, struct list_head *uf) @@ -2604,12 +2646,17 @@ static unsigned long __mmap_region(struct file *file, unsigned long addr, VMA_ITERATOR(vmi, mm, addr); MMAP_STATE(map, mm, &vmi, addr, len, pgoff, vm_flags, file); + map.check_ksm_early = can_set_ksm_flags_early(&map); + error = __mmap_prepare(&map, uf); if (!error && have_mmap_prepare) error = call_mmap_prepare(&map); if (error) goto abort_munmap; + if (map.check_ksm_early) + update_ksm_flags(&map); + /* Attempt to merge with adjacent VMAs... */ if (map.prev || map.next) { VMG_MMAP_STATE(vmg, &map, /* vma = */ NULL); @@ -2721,6 +2768,7 @@ int do_brk_flags(struct vma_iterator *vmi, struct vm_area_struct *vma, * Note: This happens *after* clearing old mappings in some code paths. */ flags |= VM_DATA_DEFAULT_FLAGS | VM_ACCOUNT | mm->def_flags; + flags = ksm_vma_flags(mm, NULL, flags); if (!may_expand_vm(mm, flags, len >> PAGE_SHIFT)) return -ENOMEM; @@ -2764,7 +2812,6 @@ int do_brk_flags(struct vma_iterator *vmi, struct vm_area_struct *vma, mm->map_count++; validate_mm(mm); - ksm_add_vma(vma); out: perf_event_mmap(vma); mm->total_vm += len >> PAGE_SHIFT; diff --git a/tools/testing/vma/vma_internal.h b/tools/testing/vma/vma_internal.h index 14718ca23a05b..0f013784da897 100644 --- a/tools/testing/vma/vma_internal.h +++ b/tools/testing/vma/vma_internal.h @@ -1484,4 +1484,15 @@ static inline void vma_set_file(struct vm_area_struct *vma, struct file *file) fput(file); } +static inline bool shmem_file(struct file *) +{ + return false; +} + +static inline vm_flags_t ksm_vma_flags(const struct mm_struct *, const struct file *, + vm_flags_t vm_flags) +{ + return vm_flags; +} + #endif /* __MM_VMA_INTERNAL_H */ -- 2.47.2