linux-kernel - UBIFS and page migration (take 3)

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Message-Id: <1462974823-3168-1-git-send-email-richard@nod.at>
Date:	Wed, 11 May 2016 15:53:41 +0200
From:	Richard Weinberger <richard@....at>
To:	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, vbabka@...e.cz
Subject: UBIFS and page migration (take 3)

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. Non-trivial block based filesystems
do not notice since they can use buffer_migrate_page().
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 the 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.

Comments? Flames? :-)

Thanks,
//richard

[PATCH 1/2] mm: Export migrate_page_move_mapping and
[PATCH 2/2] UBIFS: Implement ->migratepage()