On Mon 09-01-23 12:53:23, Suren Baghdasaryan wrote:
> Introduce lock_vma_under_rcu function to lookup and lock a VMA during
> page fault handling. When VMA is not found, can't be locked or changes
> after being locked, the function returns NULL. The lookup is performed
> under RCU protection to prevent the found VMA from being destroyed before
> the VMA lock is acquired. VMA lock statistics are updated according to
> the results.
> For now only anonymous VMAs can be searched this way. In other cases the
> function returns NULL.
Could you describe why only anonymous vmas are handled at this stage and
what (roughly) has to be done to support other vmas? lock_vma_under_rcu
doesn't seem to have any anonymous vma specific requirements AFAICS.
Also isn't lock_vma_under_rcu effectively find_read_lock_vma? Not that
the naming is really the most important part but the rcu locking is
internal to the function so why should we spread this implementation
detail to the world...
> Signed-off-by: Suren Baghdasaryan <surenb@xxxxxxxxxx>
> ---
> include/linux/mm.h | 3 +++
> mm/memory.c | 51 ++++++++++++++++++++++++++++++++++++++++++++++
> 2 files changed, 54 insertions(+)
>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index c464fc8a514c..d0fddf6a1de9 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -687,6 +687,9 @@ static inline void vma_assert_no_reader(struct vm_area_struct *vma)
> vma);
> }
>
> +struct vm_area_struct *lock_vma_under_rcu(struct mm_struct *mm,
> + unsigned long address);
> +
> #else /* CONFIG_PER_VMA_LOCK */
>
> static inline void vma_init_lock(struct vm_area_struct *vma) {}
> diff --git a/mm/memory.c b/mm/memory.c
> index 9ece18548db1..a658e26d965d 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -5242,6 +5242,57 @@ vm_fault_t handle_mm_fault(struct vm_area_struct *vma, unsigned long address,
> }
> EXPORT_SYMBOL_GPL(handle_mm_fault);
>
> +#ifdef CONFIG_PER_VMA_LOCK
> +/*
> + * Lookup and lock a VMA under RCU protection. Returned VMA is guaranteed to be
> + * stable and not isolated. If the VMA is not found or is being modified the
> + * function returns NULL.
> + */
> +struct vm_area_struct *lock_vma_under_rcu(struct mm_struct *mm,
> + unsigned long address)
> +{
> + MA_STATE(mas, &mm->mm_mt, address, address);
> + struct vm_area_struct *vma, *validate;
> +
> + rcu_read_lock();
> + vma = mas_walk(&mas);
> +retry:
> + if (!vma)
> + goto inval;
> +
> + /* Only anonymous vmas are supported for now */
> + if (!vma_is_anonymous(vma))
> + goto inval;
> +
> + if (!vma_read_trylock(vma))
> + goto inval;
> +
> + /* Check since vm_start/vm_end might change before we lock the VMA */
> + if (unlikely(address < vma->vm_start || address >= vma->vm_end)) {
> + vma_read_unlock(vma);
> + goto inval;
> + }
> +
> + /* Check if the VMA got isolated after we found it */
> + mas.index = address;
> + validate = mas_walk(&mas);
> + if (validate != vma) {
> + vma_read_unlock(vma);
> + count_vm_vma_lock_event(VMA_LOCK_MISS);
> + /* The area was replaced with another one. */
> + vma = validate;
> + goto retry;
> + }
> +
> + rcu_read_unlock();
> + return vma;
> +inval:
> + rcu_read_unlock();
> + count_vm_vma_lock_event(VMA_LOCK_ABORT);
> + return NULL;
> +}
> +#endif /* CONFIG_PER_VMA_LOCK */
> +
> #ifndef __PAGETABLE_P4D_FOLDED
> /*
> * Allocate p4d page table.
> --
> 2.39.0
--
Michal Hocko
SUSE Labs
