Re: [RFC 0/3] Recursive reclaim (on __PF_MEMALLOC)

[Peter Zijlstra]

On Tue, 2007-08-14 at 07:21 -0700, Christoph Lameter wrote:
> The following patchset implements recursive reclaim. Recursive reclaim
> is necessary if we run out of memory in the writeout patch from reclaim.
> 
> This is f.e. important for stacked filesystems or anything that does
> complicated processing in the writeout path.
> 
> Recursive reclaim works because it limits itself to only reclaim pages
> that do not require writeout. It will only remove clean pages from the LRU.
> The dirty throttling of the VM during regular reclaim insures that the amount
> of dirty pages is limited. 

No it doesn't. All memory can be tied up by anonymous pages - who are
dirty by definition and are not clamped by the dirty limit.

> If recursive reclaim causes too many clean pages
> to be removed then regular reclaim will throttle all processes until the
> dirty ratio is restored. This means that the amount of memory that can
> be reclaimed via recursive reclaim is limited to clean memory. The default
> ratio is 10%. This means that recursive reclaim can reclaim 90% of memory
> before failing. Reclaiming excessive amounts of clean pages may have a
> significant performance impact because this means that executable pages
> will be removed. However, it ensures that we will no longer fail in the
> writeout path.
> 
> A patch is included to test this functionality. The test involved allocating
> 12 Megabytes from the reclaim paths when __PF_MEMALLOC is set. This is enough
> to exhaust the reserves.
> 

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