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

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Date:	Fri, 17 Jun 2016 21:36:30 +0200
From:	Richard Weinberger <richard@....at>
To:	Michal Hocko <mhocko@...nel.org>
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()



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().

@@ -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