Re: [PATCH v9 11/17] mm: replace vm_lock and detached flag with a reference count

[Suren Baghdasaryan]

On Sun, Jan 12, 2025 at 6:25 PM Wei Yang <richard.weiyang@xxxxxxxxx> wrote:
>
> On Mon, Jan 13, 2025 at 01:47:29AM +0000, Wei Yang wrote:
> >On Sat, Jan 11, 2025 at 12:14:47PM -0800, Suren Baghdasaryan wrote:
> >>On Sat, Jan 11, 2025 at 3:24 AM Mateusz Guzik <mjguzik@xxxxxxxxx> wrote:
> >>>
> >>> On Fri, Jan 10, 2025 at 08:25:58PM -0800, Suren Baghdasaryan wrote:
> >>>
> >>> So there were quite a few iterations of the patch and I have not been
> >>> reading majority of the feedback, so it may be I missed something,
> >>> apologies upfront. :)
> >>>
> >
> >Hi, I am new to memory barriers. Hope not bothering.
> >
> >>> >  /*
> >>> >   * Try to read-lock a vma. The function is allowed to occasionally yield false
> >>> >   * locked result to avoid performance overhead, in which case we fall back to
> >>> > @@ -710,6 +742,8 @@ static inline void vma_lock_init(struct vm_area_struct *vma)
> >>> >   */
> >>> >  static inline bool vma_start_read(struct vm_area_struct *vma)
> >>> >  {
> >>> > +     int oldcnt;
> >>> > +
> >>> >       /*
> >>> >        * Check before locking. A race might cause false locked result.
> >>> >        * We can use READ_ONCE() for the mm_lock_seq here, and don't need
> >>> > @@ -720,13 +754,19 @@ static inline bool vma_start_read(struct vm_area_struct *vma)
> >>> >       if (READ_ONCE(vma->vm_lock_seq) == READ_ONCE(vma->vm_mm->mm_lock_seq.sequence))
> >>> >               return false;
> >>> >
> >>> > -     if (unlikely(down_read_trylock(&vma->vm_lock.lock) == 0))
> >>> > +     /*
> >>> > +      * If VMA_LOCK_OFFSET is set, __refcount_inc_not_zero_limited() will fail
> >>> > +      * because VMA_REF_LIMIT is less than VMA_LOCK_OFFSET.
> >>> > +      */
> >>> > +     if (unlikely(!__refcount_inc_not_zero_limited(&vma->vm_refcnt, &oldcnt,
> >>> > +                                                   VMA_REF_LIMIT)))
> >>> >               return false;
> >>> >
> >>>
> >>> Replacing down_read_trylock() with the new routine loses an acquire
> >>> fence. That alone is not a problem, but see below.
> >>
> >>Hmm. I think this acquire fence is actually necessary. We don't want
> >>the later vm_lock_seq check to be reordered and happen before we take
> >>the refcount. Otherwise this might happen:
> >>
> >>reader             writer
> >>if (vm_lock_seq == mm_lock_seq) // check got reordered
> >>        return false;
> >>                       vm_refcnt += VMA_LOCK_OFFSET
> >>                       vm_lock_seq == mm_lock_seq
> >>                       vm_refcnt -= VMA_LOCK_OFFSET
> >>if (!__refcount_inc_not_zero_limited())
> >>        return false;
> >>
> >>Both reader's checks will pass and the reader would read-lock a vma
> >>that was write-locked.
> >>
> >
> >Here what we plan to do is define __refcount_inc_not_zero_limited() with
> >acquire fence, e.g. with atomic_try_cmpxchg_acquire(), right?
> >
>
> BTW, usually we pair acquire with release.
>
> The __vma_start_write() provide release fence when locked, so for this part
> we are ok, right?

Yes, __vma_start_write() -> __vma_exit_locked() ->
refcount_sub_and_test() and this function provides release memory
ordering, see https://elixir.bootlin.com/linux/v6.12.6/source/include/linux/refcount.h#L289

>
>
> --
> Wei Yang
> Help you, Help me