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Date: Mon, 16 May 2016 14:44:49 +0200 From: Richard Weinberger <richard@....at> To: Christoph Hellwig <hch@...radead.org> Cc: linux-fsdevel@...r.kernel.org, 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, hughd@...gle.com, mgorman@...hsingularity.net, vbabka@...e.cz Subject: Re: UBIFS and page migration (take 3) Christoph, Am 12.05.2016 um 13:49 schrieb Christoph Hellwig: > Hi Richard, > > the series looks fine to me, but it fails to address the root cause: Is this a Reviewed-by? :-) > that we have an inherently dangerous default for ->migratepage that > assumes that file systems are implemented a certain way. I think the > series should also grow a third patch to remove the default and just > wire it up for the known good file systems, although we'd need some > input on what known good is. > > Any idea what filesystems do get regular testing with code that's using > CMA? A good approximation might be those that use the bufer_head > based aops from fs/buffer.c No idea how much is being tested. I fear most issues are unknown. At least for UBIFS it took years to get aware of the issue. Thanks again to Maxime and Boris for providing a reproducer. There are two classes of issues: a) filesystems that use buffer_migrate_page() but shouldn't b) filesystems that don't implement ->migratepage() and fallback_migrate_page() is not suitable. As starter we could kill the automatic assignment of fallback_migrate_page() and non-buffer_head filesystems need to figure out whether fallback_migrate_page() is suitable or not. UBIFS found out the hard way. ;-\ MM folks, do we have a way to force page migration? Maybe we can create a generic stress test. Thanks, //richard
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