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

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Date:   Tue, 17 Aug 2021 20:18:01 -0700
From:   Roman Gushchin <guro@...com>
To:     Muchun Song <songmuchun@...edance.com>
CC:     <hannes@...xchg.org>, <mhocko@...nel.org>,
        <akpm@...ux-foundation.org>, <shakeelb@...gle.com>,
        <vdavydov.dev@...il.com>, <linux-kernel@...r.kernel.org>,
        <linux-mm@...ck.org>, <duanxiongchun@...edance.com>,
        <fam.zheng@...edance.com>, <bsingharora@...il.com>,
        <shy828301@...il.com>, <alexs@...nel.org>, <smuchun@...il.com>,
        <zhengqi.arch@...edance.com>
Subject: Re: [PATCH v1 03/12] mm: memcontrol: make lruvec lock safe when LRU
 pages are reparented

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@...edance.com>
> Acked-by: Roman Gushchin <guro@...com>

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.

Thanks!