On Tue, May 24, 2022 at 03:22:55PM -0400, Johannes Weiner wrote:
> On Tue, May 24, 2022 at 02:05:42PM +0800, Muchun Song wrote:
> > If we reuse the objcg APIs to charge LRU pages, the folio_memcg()
> > can be changed when the LRU pages reparented. In this case, we need
> > to acquire the new lruvec lock.
> >
> > lruvec = folio_lruvec(folio);
> >
> > // The page is reparented.
> >
> > compact_lock_irqsave(&lruvec->lru_lock, &flags, cc);
> >
> > // Acquired the wrong lruvec lock and need to retry.
> >
> > But compact_lock_irqsave() only take lruvec lock as the parameter,
> > we cannot aware this change. If it can take the page as parameter
> > to acquire the lruvec lock. When the page memcg is changed, we can
> > use the folio_memcg() detect whether we need to reacquire the new
> > lruvec lock. So compact_lock_irqsave() is not suitable for us.
> > Similar to folio_lruvec_lock_irqsave(), introduce
> > compact_folio_lruvec_lock_irqsave() to acquire the lruvec lock in
> > the compaction routine.
> >
> > Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
>
> This looks generally good to me.
>
> It did raise the question how deferencing lruvec is safe before the
> lock is acquired when reparenting can race. The answer is in the next
> patch when you add the rcu_read_lock(). Since the patches aren't big,
> it would probably be better to merge them.
>
Will do in v5.
> > @@ -509,6 +509,29 @@ static bool compact_lock_irqsave(spinlock_t *lock, unsigned long *flags,
> > return true;
> > }
> >
> > +static struct lruvec *
> > +compact_folio_lruvec_lock_irqsave(struct folio *folio, unsigned long *flags,
> > + struct compact_control *cc)
> > +{
> > + struct lruvec *lruvec;
> > +
> > + lruvec = folio_lruvec(folio);
> > +
> > + /* Track if the lock is contended in async mode */
> > + if (cc->mode == MIGRATE_ASYNC && !cc->contended) {
> > + if (spin_trylock_irqsave(&lruvec->lru_lock, *flags))
> > + goto out;
> > +
> > + cc->contended = true;
> > + }
> > +
> > + spin_lock_irqsave(&lruvec->lru_lock, *flags);
>
> Can you implement this on top of the existing one?
>
> lruvec = folio_lruvec(folio);
> compact_lock_irqsave(&lruvec->lru_lock, flags);
> lruvec_memcg_debug(lruvec, folio);
> return lruvec;
>
I'll do a try. Thanks for your suggestions.
