linux-kernel - Re: [PATCH 41/41] mm: replace rw_semaphore with atomic_t in vma

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Message-ID: <CAJuCfpECJhUu3fvWbBzmAkEA3+1LTkKqJOVadQB_-_mEHME=xg@mail.gmail.com>
Date:   Mon, 16 Jan 2023 20:34:36 -0800
From:   Suren Baghdasaryan <surenb@...gle.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Hyeonggon Yoo <42.hyeyoo@...il.com>, akpm@...ux-foundation.org,
        michel@...pinasse.org, jglisse@...gle.com, mhocko@...e.com,
        vbabka@...e.cz, hannes@...xchg.org, mgorman@...hsingularity.net,
        dave@...olabs.net, liam.howlett@...cle.com, peterz@...radead.org,
        ldufour@...ux.ibm.com, laurent.dufour@...ibm.com,
        paulmck@...nel.org, luto@...nel.org, songliubraving@...com,
        peterx@...hat.com, david@...hat.com, dhowells@...hat.com,
        hughd@...gle.com, bigeasy@...utronix.de, kent.overstreet@...ux.dev,
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        arjunroy@...gle.com, soheil@...gle.com, hughlynch@...gle.com,
        leewalsh@...gle.com, posk@...gle.com, linux-mm@...ck.org,
        linux-arm-kernel@...ts.infradead.org,
        linuxppc-dev@...ts.ozlabs.org, x86@...nel.org,
        linux-kernel@...r.kernel.org, kernel-team@...roid.com
Subject: Re: [PATCH 41/41] mm: replace rw_semaphore with atomic_t in vma_lock

On Mon, Jan 16, 2023 at 8:14 PM Matthew Wilcox <willy@...radead.org> wrote:
>
> On Mon, Jan 16, 2023 at 11:14:38AM +0000, Hyeonggon Yoo wrote:
> > > @@ -643,20 +647,28 @@ static inline void vma_write_lock(struct vm_area_struct *vma)
> > >  static inline bool vma_read_trylock(struct vm_area_struct *vma)
> > >  {
> > >     /* Check before locking. A race might cause false locked result. */
> > > -   if (vma->vm_lock_seq == READ_ONCE(vma->vm_mm->mm_lock_seq))
> > > +   if (vma->vm_lock->lock_seq == READ_ONCE(vma->vm_mm->mm_lock_seq))
> > >             return false;
> > >
> > > -   if (unlikely(down_read_trylock(&vma->vm_lock->lock) == 0))
> > > +   if (unlikely(!atomic_inc_unless_negative(&vma->vm_lock->count)))
> > >             return false;
> > >
> > > +   /* If atomic_t overflows, restore and fail to lock. */
> > > +   if (unlikely(atomic_read(&vma->vm_lock->count) < 0)) {
> > > +           if (atomic_dec_and_test(&vma->vm_lock->count))
> > > +                   wake_up(&vma->vm_mm->vma_writer_wait);
> > > +           return false;
> > > +   }
> > > +
> > >     /*
> > >      * Overflow might produce false locked result.
> > >      * False unlocked result is impossible because we modify and check
> > >      * vma->vm_lock_seq under vma->vm_lock protection and mm->mm_lock_seq
> > >      * modification invalidates all existing locks.
> > >      */
> > > -   if (unlikely(vma->vm_lock_seq == READ_ONCE(vma->vm_mm->mm_lock_seq))) {
> > > -           up_read(&vma->vm_lock->lock);
> > > +   if (unlikely(vma->vm_lock->lock_seq == READ_ONCE(vma->vm_mm->mm_lock_seq))) {
> > > +           if (atomic_dec_and_test(&vma->vm_lock->count))
> > > +                   wake_up(&vma->vm_mm->vma_writer_wait);
> > >             return false;
> > >     }
> >
> > With this change readers can cause writers to starve.
> > What about checking waitqueue_active() before or after increasing
> > vma->vm_lock->count?
>
> I don't understand how readers can starve a writer.  Readers do
> atomic_inc_unless_negative() so a writer can always force readers
> to fail.

I think the point here was that if page faults keep occuring and they
prevent vm_lock->count from reaching 0 then a writer will be blocked
and there is no reader throttling mechanism (no max time that writer
will be waiting).