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    SubjectRe: [PATCH 28/41] mm: introduce lock_vma_under_rcu to be used from arch-specific code
    On Tue, Jan 17, 2023 at 7:47 AM Michal Hocko <mhocko@suse.com> wrote:
    >
    > 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.

    TBH I haven't spent too much time looking into file-backed page faults
    yet but a couple of tasks I can think of are:
    - Ensure that all vma->vm_ops->fault() handlers do not rely on
    mmap_lock being read-locked;
    - vma->vm_file freeing like VMA freeing will need to be done after RCU
    grace period since page fault handlers use it. This will require some
    caution because simply adding it into __vm_area_free() called via
    call_rcu() will cause corresponding fops->release() to be called
    asynchronously. I had to solve this issue with out-of-tree SPF
    implementation when asynchronously called snd_pcm_release() was
    problematic.

    I'm sure I'm missing more potential issues and maybe Matthew and
    Michel can pinpoint more things to resolve here?

    >
    > 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...

    I wanted the name to indicate that the lookup is done with no locks
    held. But I'm open to suggestions.

    >
    > > 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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