per-bdi-throttling: synchronous writepage doesn't work correctly

[Miklos Szeredi]

Hi,

It looks like bdi_thresh will always be zero if filesystem does
synchronous writepage, resulting in very poor write performance.

Hostfs (UML) is one such example, but there might be others.

The only solution I can think of is to add a set_page_writeback();
end_page_writeback() pair (or some reduced variant, that only does
the proportions magic).  But that means auditing quite a few
filesystems...

Miklos
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