The final goal is to be able to use a VMA structure without holding the mmap_sem and to be sure that the structure will not be freed in our back. The lockless use of the VMA will be done through RCU protection and thus a dedicated freeing service is required to manage it asynchronously. As reported in a 2010's thread [1], this may impact file handling when a file is still referenced while the mapping is no more there. As the final goal is to handle anonymous VMA in a speculative way and not file backed mapping, we could close and free the file pointer in a synchronous way, as soon as we are guaranteed to not use it without holding the mmap_sem. For sanity reason, in a minimal effort, the vm_file file pointer is unset once the file pointer is put. [1] https://lore.kernel.org/linux-mm/20100104182429.833180340@xxxxxxxxx/ Signed-off-by: Laurent Dufour <ldufour@xxxxxxxxxxxxx> --- include/linux/mm.h | 4 ++++ include/linux/mm_types.h | 3 +++ mm/internal.h | 27 +++++++++++++++++++++++++++ mm/mmap.c | 13 +++++++++---- 4 files changed, 43 insertions(+), 4 deletions(-) diff --git a/include/linux/mm.h b/include/linux/mm.h index f14b2c9ddfd4..f761a9c65c74 100644 --- a/include/linux/mm.h +++ b/include/linux/mm.h @@ -529,6 +529,9 @@ static inline void vma_init(struct vm_area_struct *vma, struct mm_struct *mm) vma->vm_mm = mm; vma->vm_ops = &dummy_vm_ops; INIT_LIST_HEAD(&vma->anon_vma_chain); +#ifdef CONFIG_SPECULATIVE_PAGE_FAULT + atomic_set(&vma->vm_ref_count, 1); +#endif } static inline void vma_set_anonymous(struct vm_area_struct *vma) @@ -1418,6 +1421,7 @@ static inline void INIT_VMA(struct vm_area_struct *vma) INIT_LIST_HEAD(&vma->anon_vma_chain); #ifdef CONFIG_SPECULATIVE_PAGE_FAULT seqcount_init(&vma->vm_sequence); + atomic_set(&vma->vm_ref_count, 1); #endif } diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h index 24b3f8ce9e42..6a6159e11a3f 100644 --- a/include/linux/mm_types.h +++ b/include/linux/mm_types.h @@ -285,6 +285,9 @@ struct vm_area_struct { /* linked list of VM areas per task, sorted by address */ struct vm_area_struct *vm_next, *vm_prev; +#ifdef CONFIG_SPECULATIVE_PAGE_FAULT + atomic_t vm_ref_count; +#endif struct rb_node vm_rb; /* diff --git a/mm/internal.h b/mm/internal.h index 9eeaf2b95166..302382bed406 100644 --- a/mm/internal.h +++ b/mm/internal.h @@ -40,6 +40,33 @@ void page_writeback_init(void); vm_fault_t do_swap_page(struct vm_fault *vmf); + +extern void __free_vma(struct vm_area_struct *vma); + +#ifdef CONFIG_SPECULATIVE_PAGE_FAULT +static inline void get_vma(struct vm_area_struct *vma) +{ + atomic_inc(&vma->vm_ref_count); +} + +static inline void put_vma(struct vm_area_struct *vma) +{ + if (atomic_dec_and_test(&vma->vm_ref_count)) + __free_vma(vma); +} + +#else + +static inline void get_vma(struct vm_area_struct *vma) +{ +} + +static inline void put_vma(struct vm_area_struct *vma) +{ + __free_vma(vma); +} +#endif /* CONFIG_SPECULATIVE_PAGE_FAULT */ + void free_pgtables(struct mmu_gather *tlb, struct vm_area_struct *start_vma, unsigned long floor, unsigned long ceiling); diff --git a/mm/mmap.c b/mm/mmap.c index f7f6027a7dff..c106440dcae7 100644 --- a/mm/mmap.c +++ b/mm/mmap.c @@ -188,6 +188,12 @@ static inline void mm_write_sequnlock(struct mm_struct *mm) } #endif /* CONFIG_SPECULATIVE_PAGE_FAULT */ +void __free_vma(struct vm_area_struct *vma) +{ + mpol_put(vma_policy(vma)); + vm_area_free(vma); +} + /* * Close a vm structure and free it, returning the next. */ @@ -200,8 +206,8 @@ static struct vm_area_struct *remove_vma(struct vm_area_struct *vma) vma->vm_ops->close(vma); if (vma->vm_file) fput(vma->vm_file); - mpol_put(vma_policy(vma)); - vm_area_free(vma); + vma->vm_file = NULL; + put_vma(vma); return next; } @@ -990,8 +996,7 @@ int __vma_adjust(struct vm_area_struct *vma, unsigned long start, if (next->anon_vma) anon_vma_merge(vma, next); mm->map_count--; - mpol_put(vma_policy(next)); - vm_area_free(next); + put_vma(next); /* * In mprotect's case 6 (see comments on vma_merge), * we must remove another next too. It would clutter -- 2.21.0
