commit 0bf1457f0cfca7b " mm: vmscan: do not swap anon pages just because free+file is low" causes heavy performance regression on paging

[Christian Borntraeger]

While preparing/testing some KVM on s390 patches for the next merge window (target is kvm/next which is based on 3.15-rc1) I faced a very severe performance hickup on guest paging (all anonymous memory).

All memory bound guests are in "D" state now and the system is barely unusable.

Reverting commit 0bf1457f0cfca7bc026a82323ad34bcf58ad035d
"mm: vmscan: do not swap anon pages just because free+file is low" makes the problem go away.

According to /proc/vmstat the system is now in direct reclaim almost all the time for every page fault (more than 10x more direct reclaims than kswap reclaims)
With the patch being reverted everything is fine again.

Any ideas?

Christian

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