linux-kernel - Re: [PATCH 1/3] mm: Don't blindly assign fallback_migrate

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Date:	Fri, 17 Jun 2016 22:03:46 +0200
From:	Michal Hocko <mhocko@...nel.org>
To:	Richard Weinberger <richard@....at>
Cc:	Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, linux-mtd@...ts.infradead.org,
	hannes@...xchg.org, mgorman@...hsingularity.net,
	n-horiguchi@...jp.nec.com, kirill.shutemov@...ux.intel.com,
	hughd@...gle.com, vbabka@...e.cz, adrian.hunter@...el.com,
	dedekind1@...il.com, hch@...radead.org,
	linux-fsdevel@...r.kernel.org, boris.brezillon@...e-electrons.com,
	maxime.ripard@...e-electrons.com, david@...ma-star.at,
	david@...morbit.com, alex@...tthing.co, sasha.levin@...cle.com,
	iamjoonsoo.kim@....com, rvaswani@...eaurora.org,
	tony.luck@...el.com, shailendra.capricorn@...il.com
Subject: Re: [PATCH 1/3] mm: Don't blindly assign fallback_migrate_page()

On Fri 17-06-16 21:36:30, Richard Weinberger wrote:
> 
> 
> Am 17.06.2016 um 20:27 schrieb Michal Hocko:
> > On Fri 17-06-16 18:55:45, Richard Weinberger wrote:
> >> Am 17.06.2016 um 18:28 schrieb Michal Hocko:
> >>> But doesn't this disable the page migration and so potentially reduce
> >>> the compaction success rate for the large pile of filesystems? Without
> >>> any hint about that?
> >>
> >> The WARN_ON_ONCE() is the hint. ;)
> > 
> > Right. My reply turned a different way than I meant... I meant to say
> > that there might be different regressions caused by this change without much
> > hint that a particular warning would be the smoking gun... 
> > 
> 
> Okay, what about something like that?
> That way everything works as before and we don't have regressions
> but FS maintainers will notice the WARN_ON_ONCE() and hopefully review
> whether generic_migrate_page() is really suitable.
> If so, they can set their a_ops->migratepage to generic_migrate_page().

Yes this sounds better to me. I would just be more verbose about which
a_ops is missing the migratepage callback. The WARN_ON_ONCE will not
tell us which fs is the culprit. I am not even sure the calltrace is
really helpful and maybe printk_once would be more appropriate.

	printk_once(KERN_INFO "%ps is missing migratepage callback. Please report to the respective filesystem maintainers.\n",
			mapping->a_ops);

Or print once per a_ops would be even better but that sounds like an
over engineering...
 
> @@ -771,8 +773,15 @@ static int move_to_new_page(struct page *newpage, struct page *page,
>                  * is the most common path for page migration.
>                  */
>                 rc = mapping->a_ops->migratepage(mapping, newpage, page, mode);
> -       else
> -               rc = fallback_migrate_page(mapping, newpage, page, mode);
> +       else {
> +               /*
> +                * Dear filesystem maintainer, please verify whether
> +                * generic_migrate_page() is suitable for your
> +                * filesystem, especially wrt. page flag handling.
> +                */
> +               WARN_ON_ONCE(1);
> +               rc = generic_migrate_page(mapping, newpage, page, mode);
> +       }
> 
>         /*
>          * When successful, old pagecache page->mapping must be cleared before
> 
> Thanks,
> //richard

-- 
Michal Hocko
SUSE Labs