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Subject: Re: [PATCH 1/3] mm: Don't blindly assign fallback_migrate_page()
To: Michal Hocko <mhocko@kernel.org>
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From: Richard Weinberger <richard@nod.at>
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Date: Fri, 17 Jun 2016 18:55:45 +0200
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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. ;)
But I can understand your point we'd have to communicate that change better.

> $ git grep "\.migratepage[[:space:]]*=" -- fs | wc -l
> 16
> out of
> $ git grep "struct address_space_operations[[:space:]]*[a-zA-Z0-9_]*[[:space:]]*=" -- fs | wc -l
> 87
> 
> That just seems to be too conservative for something that even not might
> be a problem, especially when considering the fallback migration code is
> there for many years with only UBIFS seeing a problem.

UBIFS is also there for many years.
It just shows that the issue is hard to hit but at least for UBIFS it is real.

> Wouldn't it be safer to contact FS developers who might have have
> similar issue and work with them to use a proper migration code?

That was the goal of this patch. Forcing the filesystem developers
to look as the WARN_ON_ONCE() triggered.

I fear just sending a mail to linux-fsdevel@vger is not enough.

Thanks,
//richard