Re: [PATCH 1/3] mm: Don't blindly assign fallback_migrate_page()

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

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