linux-kernel - Re: [Question] Should direct reclaim time be bounded?

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Date:   Wed, 24 Apr 2019 16:35:37 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Mike Kravetz <mike.kravetz@...cle.com>,
        Michal Hocko <mhocko@...nel.org>
Cc:     "linux-mm@...ck.org" <linux-mm@...ck.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Mel Gorman <mgorman@...e.de>,
        Johannes Weiner <hannes@...xchg.org>
Subject: Re: [Question] Should direct reclaim time be bounded?

On 4/23/19 6:39 PM, Mike Kravetz wrote:
>> That being said, I do not think __GFP_RETRY_MAYFAIL is wrong here. It
>> looks like there is something wrong in the reclaim going on.
> 
> Ok, I will start digging into that.  Just wanted to make sure before I got
> into it too deep.
> 
> BTW - This is very easy to reproduce.  Just try to allocate more huge pages
> than will fit into memory.  I see this 'reclaim taking forever' behavior on
> v5.1-rc5-mmotm-2019-04-19-14-53.  Looks like it was there in v5.0 as well.

I'd suspect this in should_continue_reclaim():

        /* Consider stopping depending on scan and reclaim activity */
        if (sc->gfp_mask & __GFP_RETRY_MAYFAIL) {
                /*
                 * For __GFP_RETRY_MAYFAIL allocations, stop reclaiming if the
                 * full LRU list has been scanned and we are still failing
                 * to reclaim pages. This full LRU scan is potentially
                 * expensive but a __GFP_RETRY_MAYFAIL caller really wants to succeed
                 */
                if (!nr_reclaimed && !nr_scanned)
                        return false;

And that for some reason, nr_scanned never becomes zero. But it's hard
to figure out through all the layers of functions :/