linux-kernel - Re: [PATCH] mm, vmscan: do not loop on too_many

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Message-ID: <alpine.LSU.2.11.1707240001210.2154@eggly.anvils>
Date:   Mon, 24 Jul 2017 00:03:32 -0700 (PDT)
From:   Hugh Dickins <hughd@...gle.com>
To:     Michal Hocko <mhocko@...nel.org>
cc:     Hugh Dickins <hughd@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mel Gorman <mgorman@...e.de>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        Rik van Riel <riel@...hat.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Vlastimil Babka <vbabka@...e.cz>, linux-mm@...ck.org,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm, vmscan: do not loop on too_many_isolated for ever

On Thu, 20 Jul 2017, Michal Hocko wrote:
> On Wed 19-07-17 18:54:40, Hugh Dickins wrote:
> [...]
> > You probably won't welcome getting into alternatives at this late stage;
> > but after hacking around it one way or another because of its pointless
> > lockups, I lost patience with that too_many_isolated() loop a few months
> > back (on realizing the enormous number of pages that may be isolated via
> > migrate_pages(2)), and we've been running nicely since with something like:
> > 
> > 	bool got_mutex = false;
> > 
> > 	if (unlikely(too_many_isolated(pgdat, file, sc))) {
> > 		if (mutex_lock_killable(&pgdat->too_many_isolated))
> > 			return SWAP_CLUSTER_MAX;
> > 		got_mutex = true;
> > 	}
> > 	...
> > 	if (got_mutex)
> > 		mutex_unlock(&pgdat->too_many_isolated);
> > 
> > Using a mutex to provide the intended throttling, without an infinite
> > loop or an arbitrary delay; and without having to worry (as we often did)
> > about whether those numbers in too_many_isolated() are really appropriate.
> > No premature OOMs complained of yet.
> > 
> > But that was on a different kernel, and there I did have to make sure
> > that PF_MEMALLOC always prevented us from nesting: I'm not certain of
> > that in the current kernel (but do remember Johannes changing the memcg
> > end to make it use PF_MEMALLOC too).  I offer the preview above, to see
> > if you're interested in that alternative: if you are, then I'll go ahead
> > and make it into an actual patch against v4.13-rc.
> 
> I would rather get rid of any additional locking here and my ultimate
> goal is to make throttling at the page allocator layer rather than
> inside the reclaim.

Fair enough, I'm certainly in no hurry to send the patch,
but thought it worth mentioning.

Hugh