Re: mmap dirty limits on 32 bit kernels (Was: [BUG] New KernelBugs)

[Linus Torvalds]

On Thu, 15 Nov 2007, Linus Torvalds wrote:
> 
> The problem with HIGHMEM is that it causes various metadata (dentries, 
> inodes, page struct tables etc) to eat up memory "prime real estate" under 
> the same kind of conditions that also dirty a lot of memory. So the reason 
> we disallow HIGHMEM from dirty limits is only *partly* the per-device or 
> mapping DMA limits, and to a large degree the fact that non-highmem memory 
> is special in general, and it is usually the non-highmem areas that are 
> constrained - and need to be protected.

Final note on this (promise): 

I'd really be very interested to hear if the patch I *do* think makes 
sense (ie the removal of the old "unmapped_ratio" logic) actually already 
solves most of Bron's problems.

It may well be that that unmapped_ratio logic effectively undid the system 
configuration changes that Bron has done. It doesn't matter if Bron has

	>From our sysctl.conf:
	# This should help reduce flushing on Cache::FastMmap files
	vm.dirty_background_ratio = 50
	vm.dirty_expire_centisecs = 9000
	vm.dirty_ratio = 80
	vm.dirty_writeback_centisecs = 3000

if it turns out that the "unmapped_ratio" logic turns the 80% back down to 
5%.

It may well be that 80% of the non-highmem memory is plenty good enough! 

Sure, older kernels allowed even more of memory to be dirty (since they 
didn't count dirty mappings at all), but we may have a case where the fact 
that we discount the HIGHMEM stuff isn't the major problem in itself, and 
that the dirty_ratio sysctl should be ok - but just gets screwed over by 
that unmapped_ratio logic.

So Bron, if you can test that patch, I'd love to hear if it matters. It 
may not make any difference (maybe you don't actually trigger the 
unmapped_ratio logic at all), but I think it has the potential for being 
totally broken for you.

People that don't change the dirty_ratio from the default values would 
generally never care, because the default dirty-ratio is *already* so low 
that even if the unmapped_ratio logic triggers, it won't much matter!

		Linus
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