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

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Message-ID: <f6e25e52-bb02-6d79-b9fd-3acc8358ec45@oracle.com>
Date:   Wed, 31 Jul 2019 14:11:00 -0700
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     Vlastimil Babka <vbabka@...e.cz>, 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 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.
-- 
Mike Kravetz