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SubjectRe: [PATCH v12 19/31] mm: protect the RB tree with a sequence lock
On Tue, Apr 16, 2019 at 03:45:10PM +0200, Laurent Dufour wrote:
> Introducing a per mm_struct seqlock, mm_seq field, to protect the changes
> made in the MM RB tree. This allows to walk the RB tree without grabbing
> the mmap_sem, and on the walk is done to double check that sequence counter
> was stable during the walk.
>
> The mm seqlock is held while inserting and removing entries into the MM RB
> tree. Later in this series, it will be check when looking for a VMA
> without holding the mmap_sem.
>
> This is based on the initial work from Peter Zijlstra:
> https://lore.kernel.org/linux-mm/20100104182813.479668508@chello.nl/
>
> Signed-off-by: Laurent Dufour <ldufour@linux.ibm.com>

Reviewed-by: Jérôme Glisse <jglisse@redhat.com>

> ---
> include/linux/mm_types.h | 3 +++
> kernel/fork.c | 3 +++
> mm/init-mm.c | 3 +++
> mm/mmap.c | 48 +++++++++++++++++++++++++++++++---------
> 4 files changed, 46 insertions(+), 11 deletions(-)
>
> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> index e78f72eb2576..24b3f8ce9e42 100644
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -358,6 +358,9 @@ struct mm_struct {
> struct {
> struct vm_area_struct *mmap; /* list of VMAs */
> struct rb_root mm_rb;
> +#ifdef CONFIG_SPECULATIVE_PAGE_FAULT
> + seqlock_t mm_seq;
> +#endif
> u64 vmacache_seqnum; /* per-thread vmacache */
> #ifdef CONFIG_MMU
> unsigned long (*get_unmapped_area) (struct file *filp,
> diff --git a/kernel/fork.c b/kernel/fork.c
> index 2992d2c95256..3a1739197ebc 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -1008,6 +1008,9 @@ static struct mm_struct *mm_init(struct mm_struct *mm, struct task_struct *p,
> mm->mmap = NULL;
> mm->mm_rb = RB_ROOT;
> mm->vmacache_seqnum = 0;
> +#ifdef CONFIG_SPECULATIVE_PAGE_FAULT
> + seqlock_init(&mm->mm_seq);
> +#endif
> atomic_set(&mm->mm_users, 1);
> atomic_set(&mm->mm_count, 1);
> init_rwsem(&mm->mmap_sem);
> diff --git a/mm/init-mm.c b/mm/init-mm.c
> index a787a319211e..69346b883a4e 100644
> --- a/mm/init-mm.c
> +++ b/mm/init-mm.c
> @@ -27,6 +27,9 @@
> */
> struct mm_struct init_mm = {
> .mm_rb = RB_ROOT,
> +#ifdef CONFIG_SPECULATIVE_PAGE_FAULT
> + .mm_seq = __SEQLOCK_UNLOCKED(init_mm.mm_seq),
> +#endif
> .pgd = swapper_pg_dir,
> .mm_users = ATOMIC_INIT(2),
> .mm_count = ATOMIC_INIT(1),
> diff --git a/mm/mmap.c b/mm/mmap.c
> index 13460b38b0fb..f7f6027a7dff 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -170,6 +170,24 @@ void unlink_file_vma(struct vm_area_struct *vma)
> }
> }
>
> +#ifdef CONFIG_SPECULATIVE_PAGE_FAULT
> +static inline void mm_write_seqlock(struct mm_struct *mm)
> +{
> + write_seqlock(&mm->mm_seq);
> +}
> +static inline void mm_write_sequnlock(struct mm_struct *mm)
> +{
> + write_sequnlock(&mm->mm_seq);
> +}
> +#else
> +static inline void mm_write_seqlock(struct mm_struct *mm)
> +{
> +}
> +static inline void mm_write_sequnlock(struct mm_struct *mm)
> +{
> +}
> +#endif /* CONFIG_SPECULATIVE_PAGE_FAULT */
> +
> /*
> * Close a vm structure and free it, returning the next.
> */
> @@ -445,26 +463,32 @@ static void vma_gap_update(struct vm_area_struct *vma)
> }
>
> static inline void vma_rb_insert(struct vm_area_struct *vma,
> - struct rb_root *root)
> + struct mm_struct *mm)
> {
> + struct rb_root *root = &mm->mm_rb;
> +
> /* All rb_subtree_gap values must be consistent prior to insertion */
> validate_mm_rb(root, NULL);
>
> rb_insert_augmented(&vma->vm_rb, root, &vma_gap_callbacks);
> }
>
> -static void __vma_rb_erase(struct vm_area_struct *vma, struct rb_root *root)
> +static void __vma_rb_erase(struct vm_area_struct *vma, struct mm_struct *mm)
> {
> + struct rb_root *root = &mm->mm_rb;
> +
> /*
> * Note rb_erase_augmented is a fairly large inline function,
> * so make sure we instantiate it only once with our desired
> * augmented rbtree callbacks.
> */
> + mm_write_seqlock(mm);
> rb_erase_augmented(&vma->vm_rb, root, &vma_gap_callbacks);
> + mm_write_sequnlock(mm); /* wmb */
> }
>
> static __always_inline void vma_rb_erase_ignore(struct vm_area_struct *vma,
> - struct rb_root *root,
> + struct mm_struct *mm,
> struct vm_area_struct *ignore)
> {
> /*
> @@ -472,21 +496,21 @@ static __always_inline void vma_rb_erase_ignore(struct vm_area_struct *vma,
> * with the possible exception of the "next" vma being erased if
> * next->vm_start was reduced.
> */
> - validate_mm_rb(root, ignore);
> + validate_mm_rb(&mm->mm_rb, ignore);
>
> - __vma_rb_erase(vma, root);
> + __vma_rb_erase(vma, mm);
> }
>
> static __always_inline void vma_rb_erase(struct vm_area_struct *vma,
> - struct rb_root *root)
> + struct mm_struct *mm)
> {
> /*
> * All rb_subtree_gap values must be consistent prior to erase,
> * with the possible exception of the vma being erased.
> */
> - validate_mm_rb(root, vma);
> + validate_mm_rb(&mm->mm_rb, vma);
>
> - __vma_rb_erase(vma, root);
> + __vma_rb_erase(vma, mm);
> }
>
> /*
> @@ -601,10 +625,12 @@ void __vma_link_rb(struct mm_struct *mm, struct vm_area_struct *vma,
> * immediately update the gap to the correct value. Finally we
> * rebalance the rbtree after all augmented values have been set.
> */
> + mm_write_seqlock(mm);
> rb_link_node(&vma->vm_rb, rb_parent, rb_link);
> vma->rb_subtree_gap = 0;
> vma_gap_update(vma);
> - vma_rb_insert(vma, &mm->mm_rb);
> + vma_rb_insert(vma, mm);
> + mm_write_sequnlock(mm);
> }
>
> static void __vma_link_file(struct vm_area_struct *vma)
> @@ -680,7 +706,7 @@ static __always_inline void __vma_unlink_common(struct mm_struct *mm,
> {
> struct vm_area_struct *next;
>
> - vma_rb_erase_ignore(vma, &mm->mm_rb, ignore);
> + vma_rb_erase_ignore(vma, mm, ignore);
> next = vma->vm_next;
> if (has_prev)
> prev->vm_next = next;
> @@ -2674,7 +2700,7 @@ detach_vmas_to_be_unmapped(struct mm_struct *mm, struct vm_area_struct *vma,
> insertion_point = (prev ? &prev->vm_next : &mm->mmap);
> vma->vm_prev = NULL;
> do {
> - vma_rb_erase(vma, &mm->mm_rb);
> + vma_rb_erase(vma, mm);
> mm->map_count--;
> tail_vma = vma;
> vma = vma->vm_next;
> --
> 2.21.0
>

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