linux-kernel - Re: [RFC PATCH 1/3] mm, reclaim: make should_continue

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Message-ID: <60b15fed-2110-c783-d48c-20a1d45f354d@suse.cz>
Date:   Thu, 1 Aug 2019 10:44:44 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Mike Kravetz <mike.kravetz@...cle.com>,
        Hillf Danton <hdanton@...a.com>, Mel Gorman <mgorman@...e.de>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Michal Hocko <mhocko@...nel.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [RFC PATCH 1/3] mm, reclaim: make should_continue_reclaim perform
 dryrun detection

On 7/31/19 11:11 PM, Mike Kravetz wrote:
> On 7/31/19 4:08 AM, Vlastimil Babka wrote:
>>
>> I agree this is an improvement overall, but perhaps the patch does too
>> many things at once. The reshuffle is one thing and makes sense. The
>> change of the last return condition could perhaps be separate. Also
>> AFAICS the ultimate result is that when nr_reclaimed == 0, the function
>> will now always return false. Which makes the initial test for
>> __GFP_RETRY_MAYFAIL and the comments there misleading. There will no
>> longer be a full LRU scan guaranteed - as long as the scanned LRU chunk
>> yields no reclaimed page, we abort.
> 
> Can someone help me understand why nr_scanned == 0 guarantees a full
> LRU scan?  FWICS, nr_scanned used in this context is only incremented
> in shrink_page_list and potentially shrink_zones.  In the stall case I
> am looking at, there are MANY cases in which nr_scanned is only a few
> pages and none of those are reclaimed.
> 
> Can we not get nr_scanned == 0 on an arbitrary chunk of the LRU?
> 
> I must be missing something, because I do not see how nr_scanned == 0
> guarantees a full scan.

Yeah, seems like it doesn't. More reasons to update/remove the comment.
Can be a followup cleanup if you don't want to block the series.