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Date:   Mon, 21 Dec 2020 19:49:22 -0800 (PST)
From:   Hugh Dickins <hughd@...gle.com>
To:     Alex Shi <alex.shi@...ux.alibaba.com>
cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Hugh Dickins <hughd@...gle.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Alexander Duyck <alexander.duyck@...il.com>,
        Hui Su <sh_def@....com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/3] mm/compaction: remove rcu_read_lock during page
 compaction

On Thu, 17 Dec 2020, Alex Shi wrote:

> rcu_read_lock was used to guard memcg destory, now TestClearPageLRU
> could block this happen, so we don't need it. Remove it to reduce
> locking load in debugging mode.

"
isolate_migratepages_block() used rcu_read_lock() with the intention
of safeguarding against the mem_cgroup being destroyed concurrently;
but its TestClearPageLRU already protects against that.  Delete the
unnecessary rcu_read_lock() and _unlock().
"

> 
> Signed-off-by: Alex Shi <alex.shi@...ux.alibaba.com>
> Cc: Hugh Dickins <hughd@...gle.com>

Acked-by: Hugh Dickins <hughd@...gle.com>

> Cc: Johannes Weiner <hannes@...xchg.org>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: linux-mm@...ck.org
> Cc: linux-kernel@...r.kernel.org
> ---
>  mm/compaction.c | 5 +----
>  1 file changed, 1 insertion(+), 4 deletions(-)
> 
> diff --git a/mm/compaction.c b/mm/compaction.c
> index 8049d3530812..02af220fb992 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -995,7 +995,6 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
>  		if (!TestClearPageLRU(page))
>  			goto isolate_fail_put;
>  
> -		rcu_read_lock();
>  		lruvec = mem_cgroup_page_lruvec(page, pgdat);
>  
>  		/* If we already hold the lock, we can skip some rechecking */
> @@ -1005,7 +1004,6 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
>  
>  			compact_lock_irqsave(&lruvec->lru_lock, &flags, cc);
>  			locked = lruvec;
> -			rcu_read_unlock();
>  
>  			lruvec_memcg_debug(lruvec, page);
>  
> @@ -1026,8 +1024,7 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
>  				SetPageLRU(page);
>  				goto isolate_fail_put;
>  			}
> -		} else
> -			rcu_read_unlock();
> +		}
>  
>  		/* The whole page is taken off the LRU; skip the tail pages. */
>  		if (PageCompound(page))
> -- 
> 2.29.GIT

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