linux-kernel - Re: UBIFS and page migration (take 2)

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Message-ID: <56FE49CD.1000302@suse.cz>
Date:	Fri, 1 Apr 2016 12:13:33 +0200
From:	Vlastimil Babka <vbabka@...e.cz>
To:	Richard Weinberger <richard@....at>, linux-fsdevel@...r.kernel.org
Cc:	linux-mtd@...ts.infradead.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, boris.brezillon@...e-electrons.com,
	maxime.ripard@...e-electrons.com, david@...ma-star.at,
	david@...morbit.com, dedekind1@...il.com, alex@...tthing.co,
	akpm@...ux-foundation.org, sasha.levin@...cle.com,
	iamjoonsoo.kim@....com, rvaswani@...eaurora.org,
	tony.luck@...el.com, shailendra.capricorn@...il.com,
	kirill.shutemov@...ux.intel.com, hch@...radead.org,
	hughd@...gle.com, mgorman@...hsingularity.net
Subject: Re: UBIFS and page migration (take 2)

On 03/31/2016 11:58 PM, Richard Weinberger wrote:
> During page migrations UBIFS gets confused. We triggered this by using CMA
> on two different targets.
> It turned out that fallback_migrate_page() is not suitable for UBIFS as it
> does not copy the PagePrivate flag.
> UBIFS is using this flag among with PageChecked to account free space.
> One possible solution is implementing a ->migratepage() function in UBIFS
> which does more or less the same as fallback_migrate_page() but also
> copies PagePrivate. I'm not at all sure whether this is they way to go.
> IMHO either page migration should not happen if ->migratepage() is not implement
> or fallback_migrate_page() has to work for all filesystems.

Yes, we could document more thoroughly the expectations of 
fallback_migrate_page() and audit the existing users, but still relying on every 
new address_space_operations instance to verify them isn't without risk. And I 
doubt there can be a default fallback that's guaranteed safe for all filesystems.

> Comments? Flames? :-)
>
> Thanks,
> //richard
>
> [PATCH 1/2] mm: Export migrate_page_move_mapping and
> [PATCH 2/2] UBIFS: Implement ->migratepage()
>