linux-kernel - Re: [PATCH] mm: be more verbose for alloc_contig

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Message-ID: <YC6TpqT26dSy11fU@google.com>
Date:   Thu, 18 Feb 2021 08:19:50 -0800
From:   Minchan Kim <minchan@...nel.org>
To:     David Hildenbrand <david@...hat.com>
Cc:     Michal Hocko <mhocko@...e.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-mm <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>, joaodias@...gle.com
Subject: Re: [PATCH] mm: be more verbose for alloc_contig_range faliures

On Thu, Feb 18, 2021 at 10:43:21AM +0100, David Hildenbrand wrote:
> On 18.02.21 10:35, Michal Hocko wrote:
> > On Thu 18-02-21 10:02:43, David Hildenbrand wrote:
> > > On 18.02.21 09:56, Michal Hocko wrote:
> > > > On Wed 17-02-21 08:36:03, Minchan Kim wrote:
> > > > > alloc_contig_range is usually used on cma area or movable zone.
> > > > > It's critical if the page migration fails on those areas so
> > > > > dump more debugging message like memory_hotplug unless user
> > > > > specifiy __GFP_NOWARN.
> > > > 
> > > > I agree with David that this has a potential to generate a lot of output
> > > > and it is not really clear whether it is worth it. Page isolation code
> > > > already has REPORT_FAILURE mode which currently used only for the memory
> > > > hotplug because this was just too noisy from the CMA path - d381c54760dc
> > > > ("mm: only report isolation failures when offlining memory").
> > > > 
> > > > Maybe migration failures are less likely to fail but still.
> > > 
> > > Side note: I really dislike that uncontrolled error reporting on memory
> > > offlining path we have enabled as default. Yeah, it might be useful for
> > > ZONE_MOVABLE in some cases, but otherwise it's just noise.
> > > 
> > > Just do a "sudo stress-ng --memhotplug 1" and see the log getting flooded
> > 
> > Anyway we can discuss this in a separate thread but I think this is not
> > a representative workload.
> 
> Sure, but the essence is "this is noise", and we'll have more noise on
> alloc_contig_range() as we see these calls more frequently. There should be
> an explicit way to enable such *debug* messages.

alloc_contig_range already has gfp_mask and it respects __GFP_NOWARN.
Why shouldn't people use it if they don't care the failure?
Semantically, it makes sense to me.

About the messeage flooding, shouldn't we go with ratelimiting?
I see those two problem are orthgonal.