Re: [PATCH v1 03/12] mm: memcontrol: make lruvec lock safe when LRU pages are reparented

[Muchun Song]

On Wed, Aug 18, 2021 at 11:18 AM Roman Gushchin <guro@xxxxxx> wrote:
>
> On Sat, Aug 14, 2021 at 01:25:10PM +0800, Muchun Song wrote:
> > The diagram below shows how to make the folio lruvec lock safe when LRU
> > pages are reparented.
> >
> > folio_lruvec_lock(folio)
> >     retry:
> >       lruvec = folio_lruvec(folio);
> >
> >         // The folio is reparented at this time.
> >         spin_lock(&lruvec->lru_lock);
> >
> >         if (unlikely(lruvec_memcg(lruvec) != folio_memcg(folio)))
> >             // Acquired the wrong lruvec lock and need to retry.
> >             // Because this folio is on the parent memcg lruvec list.
> >             goto retry;
> >
> >         // If we reach here, it means that folio_memcg(folio) is stable.
> >
> > memcg_reparent_objcgs(memcg)
> >     // lruvec belongs to memcg and lruvec_parent belongs to parent memcg.
> >     spin_lock(&lruvec->lru_lock);
> >     spin_lock(&lruvec_parent->lru_lock);
> >
> >     // Move all the pages from the lruvec list to the parent lruvec list.
> >
> >     spin_unlock(&lruvec_parent->lru_lock);
> >     spin_unlock(&lruvec->lru_lock);
> >
> > After we acquire the lruvec lock, we need to check whether the folio is
> > reparented. If so, we need to reacquire the new lruvec lock. On the
> > routine of the LRU pages reparenting, we will also acquire the lruvec
> > lock (will be implemented in the later patch). So folio_memcg() cannot
> > be changed when we hold the lruvec lock.
> >
> > Since lruvec_memcg(lruvec) is always equal to folio_memcg(folio) after
> > we hold the lruvec lock, lruvec_memcg_debug() check is pointless. So
> > remove it.
> >
> > This is a preparation for reparenting the LRU pages.
> >
> > Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
> > Acked-by: Roman Gushchin <guro@xxxxxx>
>
> Maybe it's mostly s/page/folio, but the patch looks quite differently
> in comparison to the version I did ack. In general, please, drop acks
> when there are significant changes between versions.

Got it. I'll drop all acks in the next version. Thanks.

>
> Thanks!