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    SubjectRe: [PATCH 28/41] mm: introduce lock_vma_under_rcu to be used from arch-specific code
    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@google.com>
    > ---
    > 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

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