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Date:   Sat, 19 Dec 2020 01:41:07 +0800
From:   Jacob Wen <jian.w.wen@...cle.com>
To:     Michal Hocko <mhocko@...e.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        akpm@...ux-foundation.org
Subject: Re: [PATCH] mm/vmscan: DRY cleanup for do_try_to_free_pages()


On 12/18/20 10:27 PM, Michal Hocko wrote:
> On Fri 18-12-20 21:51:48, Jacob Wen wrote:
>> On 12/18/20 6:51 PM, Michal Hocko wrote:
>>> On Fri 18-12-20 18:22:17, Jacob Wen wrote:
>>>> This patch reduces repetition of set_task_reclaim_state() around
>>>> do_try_to_free_pages().
>>> The changelog really should be talking about why this is needed/useful.
>>>   From the above it is not really clear whether you aimed at doing
>>> a clean up or this is a fix for some misbehavior. I do assume the former
>>> but this should be clearly articulated.
>> How about this?
>>
>> mm/vmscan: remove duplicate code around do_try_to_free_pages()
>>
>> This patch moves set_task_reclaim_state() into do_try_to_free_pages()
>> to avoid unnecessary repetition. It doesn't introduce functional
>> change.
> This is still more about what is changed more than why it is changed. I
> would go with something like the following:
> "
> reclaim_state has to be set for all reclaim paths because it acts as a
> storage to collect reclaim feedback. Currently set_task_reclaim_state is
> called from each highlevel reclaim function. Simplify the code flow by
> moving set_task_reclaim_state into core direct reclaim function
> (do_try_to_free_pages) for all direct reclaim paths.
> "
>
> To the patch itself. I am not opposed but I do not see an urgent reason
> to take it either. The net LOC increases slightly, it makes
> do_try_to_free_pages slightly more tricky due to different early return
> paths. Highlevel direct reclaim functions do not tend to change a lot.

set_task_reclaim_state() is a function with 3 lines of code of which 2 
lines contain WARN_ON_ONCE.

I am not comfortable with the current repetition.

>
>>>> Signed-off-by: Jacob Wen <jian.w.wen@...cle.com>
>>>> ---
>>>>    mm/vmscan.c | 27 ++++++++++++++++-----------
>>>>    1 file changed, 16 insertions(+), 11 deletions(-)
>>>>
>>>> diff --git a/mm/vmscan.c b/mm/vmscan.c
>>>> index 257cba79a96d..4bc244b23686 100644
>>>> --- a/mm/vmscan.c
>>>> +++ b/mm/vmscan.c
>>>> @@ -3023,6 +3023,10 @@ static unsigned long do_try_to_free_pages(struct zonelist *zonelist,
>>>>    	pg_data_t *last_pgdat;
>>>>    	struct zoneref *z;
>>>>    	struct zone *zone;
>>>> +	unsigned long ret;
>>>> +
>>>> +	set_task_reclaim_state(current, &sc->reclaim_state);
>>>> +
>>>>    retry:
>>>>    	delayacct_freepages_start();
>>>> @@ -3069,12 +3073,16 @@ static unsigned long do_try_to_free_pages(struct zonelist *zonelist,
>>>>    	delayacct_freepages_end();
>>>> -	if (sc->nr_reclaimed)
>>>> -		return sc->nr_reclaimed;
>>>> +	if (sc->nr_reclaimed) {
>>>> +		ret = sc->nr_reclaimed;
>>>> +		goto out;
>>>> +	}
>>>>    	/* Aborted reclaim to try compaction? don't OOM, then */
>>>> -	if (sc->compaction_ready)
>>>> -		return 1;
>>>> +	if (sc->compaction_ready) {
>>>> +		ret = 1;
>>>> +		goto out;
>>>> +	}
>>>>    	/*
>>>>    	 * We make inactive:active ratio decisions based on the node's
>>>> @@ -3101,7 +3109,10 @@ static unsigned long do_try_to_free_pages(struct zonelist *zonelist,
>>>>    		goto retry;
>>>>    	}
>>>> -	return 0;
>>>> +	ret = 0;
>>>> +out:
>>>> +	set_task_reclaim_state(current, NULL);
>>>> +	return ret;
>>>>    }
>>>>    static bool allow_direct_reclaim(pg_data_t *pgdat)
>>>> @@ -3269,13 +3280,11 @@ unsigned long try_to_free_pages(struct zonelist *zonelist, int order,
>>>>    	if (throttle_direct_reclaim(sc.gfp_mask, zonelist, nodemask))
>>>>    		return 1;
>>>> -	set_task_reclaim_state(current, &sc.reclaim_state);
>>>>    	trace_mm_vmscan_direct_reclaim_begin(order, sc.gfp_mask);
>>>>    	nr_reclaimed = do_try_to_free_pages(zonelist, &sc);
>>>>    	trace_mm_vmscan_direct_reclaim_end(nr_reclaimed);
>>>> -	set_task_reclaim_state(current, NULL);
>>>>    	return nr_reclaimed;
>>>>    }
>>>> @@ -3347,7 +3356,6 @@ unsigned long try_to_free_mem_cgroup_pages(struct mem_cgroup *memcg,
>>>>    	 */
>>>>    	struct zonelist *zonelist = node_zonelist(numa_node_id(), sc.gfp_mask);
>>>> -	set_task_reclaim_state(current, &sc.reclaim_state);
>>>>    	trace_mm_vmscan_memcg_reclaim_begin(0, sc.gfp_mask);
>>>>    	noreclaim_flag = memalloc_noreclaim_save();
>>>> @@ -3355,7 +3363,6 @@ unsigned long try_to_free_mem_cgroup_pages(struct mem_cgroup *memcg,
>>>>    	memalloc_noreclaim_restore(noreclaim_flag);
>>>>    	trace_mm_vmscan_memcg_reclaim_end(nr_reclaimed);
>>>> -	set_task_reclaim_state(current, NULL);
>>>>    	return nr_reclaimed;
>>>>    }
>>>> @@ -4023,11 +4030,9 @@ unsigned long shrink_all_memory(unsigned long nr_to_reclaim)
>>>>    	fs_reclaim_acquire(sc.gfp_mask);
>>>>    	noreclaim_flag = memalloc_noreclaim_save();
>>>> -	set_task_reclaim_state(current, &sc.reclaim_state);
>>>>    	nr_reclaimed = do_try_to_free_pages(zonelist, &sc);
>>>> -	set_task_reclaim_state(current, NULL);
>>>>    	memalloc_noreclaim_restore(noreclaim_flag);
>>>>    	fs_reclaim_release(sc.gfp_mask);
>>>> -- 
>>>> 2.25.1
>>>>

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