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

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Date:   Wed, 17 Feb 2021 09:26:14 -0800
From:   Minchan Kim <minchan@...nel.org>
To:     David Hildenbrand <david@...hat.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        linux-mm <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>, mhocko@...e.com,
        joaodias@...gle.com
Subject: Re: [PATCH] mm: be more verbose for alloc_contig_range faliures

On Wed, Feb 17, 2021 at 05:51:27PM +0100, David Hildenbrand wrote:
> On 17.02.21 17:36, 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.
> > 
> > Signed-off-by: Minchan Kim <minchan@...nel.org>
> > ---
> >   mm/page_alloc.c | 16 +++++++++++++++-
> >   1 file changed, 15 insertions(+), 1 deletion(-)
> > 
> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index 0b55c9c95364..67f3ee3a1528 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -8486,6 +8486,15 @@ static int __alloc_contig_migrate_range(struct compact_control *cc,
> >   				NULL, (unsigned long)&mtc, cc->mode, MR_CONTIG_RANGE);
> >   	}
> >   	if (ret < 0) {
> > +		if (!(cc->gfp_mask & __GFP_NOWARN)) {
> > +			struct page *page;
> > +
> > +			list_for_each_entry(page, &cc->migratepages, lru) {
> > +				pr_warn("migrating pfn %lx failed ret:%d ",
> > +						page_to_pfn(page), ret);
> > +				dump_page(page, "migration failure");
> > +			}
> 
> This can create *a lot* of noise. For example, until huge pages are actually
> considered, we will choke on each end every huge page - and might do so over
> and over again.

I am not familiar with huge page status at this moment but why couldn't
they use __GFP_NOWARN if they are supposed to fail frequently?

> 
> This might be helpful for debugging, but is unacceptable for production
> systems for now I think. Maybe for now, do it based on CONFIG_DEBUG_VM.

If it's due to huge page you mentioned above and caller passes
__GFP_NOWARN in that case, couldn't we enable always-on?

Actually, I am targeting cma allocation failure, which should
be rather rare compared to other call sites but critical to fail.
If it's concern to emit too many warning message, I will scope
down for site for only cma allocation.

> 
> > +		}
> >   		putback_movable_pages(&cc->migratepages);
> >   		return ret;
> >   	}
> > @@ -8728,6 +8737,8 @@ struct page *alloc_contig_pages(unsigned long nr_pages, gfp_t gfp_mask,
> >   		pfn = ALIGN(zone->zone_start_pfn, nr_pages);
> >   		while (zone_spans_last_pfn(zone, pfn, nr_pages)) {
> >   			if (pfn_range_valid_contig(zone, pfn, nr_pages)) {
> > +				unsigned long gfp_flags;
> > +
> >   				/*
> >   				 * We release the zone lock here because
> >   				 * alloc_contig_range() will also lock the zone
> > @@ -8736,8 +8747,11 @@ struct page *alloc_contig_pages(unsigned long nr_pages, gfp_t gfp_mask,
> >   				 * and cause alloc_contig_range() to fail...
> >   				 */
> >   				spin_unlock_irqrestore(&zone->lock, flags);
> > +
> > +				if (zone_idx(zone) != ZONE_MOVABLE)
> > +					gfp_flags = gfp_mask | __GFP_NOWARN;
> 
> This feels wrong. It might be better to make that decision inside
> __alloc_contig_migrate_range() based on cc->zone.

CMA could be any normal zone and the suggestion will make it slient.