3.6-rc7 32-bit PAE miscalculates dirty page limits

[Pierre Beck]

Hi,

there seems to be a bug in either ext4 or VM code triggered with 16 GB 
memory when compiled with 32-bit and PAE. dirty_background_ratio 
defaults to 10, dirty_ratio to 20. But in effect, dirty pages are 
strongly limited (zero or negative?). I observed extreme I/O wait states 
and slow disk access. A quick cure was to set dirty_bytes and 
dirty_background_bytes to sane values, overriding the ratios. An 
educated guess: the result of dirty_ratio calculation is stored as an 
unsigned 32-bit integer and overflows?

Greetings,

Pierre Beck
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