Those pages are useless! It's silly having slab hanging onto them
while we go and reclaim useful pagecache instead.
I *really* think we need to throw away those pages instantly.
The only possible reason for hanging onto them is because they're
cache-warm. And we need a global-scope cpu-local hot pages queue
anyway.
And once we have that, slab _must_ release its warm pages into it.
It's counterproductive for slab to hang onto warm pages when, say,
a pagefault needs one.
> It could also be that by the time the
> pages are reclaimed another <n> have been cleaned. IMO its no worst than
> have freeable pages on lru from any other source. If we get close to oom
> we will call kmem_cache_reap, otherwise we let the lru find the pages.
As I say, by not releasing those (useless to slab) pages, we're causing
other (useful) stuff to be reclaimed.
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