Re: [PATCH] KVM: MMU: Fix mmu_shrink() so that it can free mmu pages as intended

[Takuya Yoshikawa <takuya.yoshikawa@gmail.com>]

On Wed, 18 Jul 2012 17:52:46 -0300
Marcelo Tosatti <mtosatti@redhat.com> wrote:

> Can't understand, can you please expand more clearly? 

I think mmu pages are not worth freeing under usual memory pressure,
especially when we have EPT/NPT on.

What's happening:
shrink_slab() vainly calls mmu_shrink() with the default batch size 128,
and mmu_shrink() takes a long time to zap mmu pages far fewer than the
requested number, usually just frees one.  Sadly, KVM may recreate the
page soon after that.

Since we set the seeks 10 times greater than the default, total_scan is
very small and shrink_slab() just wastes time for freeing such small
amount of may-be-reallocated-soon memory: I want it to use time for
scanning other objects instead.

Actually the total amount of memory used for mmu pages is not huge in
the case of EPT/NPT on: maybe smaller that that of rmap?


So, it's clear that no one wants mmu pages to be freed as other objects.
Sure, our seeks size prevents shrink_slab() from calling mmu_shrink()
usually.  But what if administrators want to drop clean caches on the
host?

Documentation/sysctl/vm.txt says:
  Writing to this will cause the kernel to drop clean caches, dentries and
  inodes from memory, causing that memory to become free.

  To free pagecache:
          echo 1 > /proc/sys/vm/drop_caches
  To free dentries and inodes:
          echo 2 > /proc/sys/vm/drop_caches
  To free pagecache, dentries and inodes:
          echo 3 > /proc/sys/vm/drop_caches

I don't want mmu pages to be freed in such cases.


So, how about stopping reporting/returning the total number of used
mmu pages to shrink_slab()?

If we do so, it will think that there are not enough objects to get
memory back from KVM.

In the case of shadow paging, guests can do bad things to allocate
enormous mmu pages, so we should report such exceeded numbers to
shrink_slab() as freeable objects, not the total.

  |--- needed ---|--- freeable under memory pressure ---|

We may be able to use n_max_mmu_pages for this: the shrinker tries
to free mmu pages unless the number reaches the goal.

Thanks,
	Takuya