Re: [patch 10/10] mm: workingset: keep shadow entries in check

From: Johannes Weiner
Date: Mon Jun 03 2013 - 11:21:18 EST

On Mon, Jun 03, 2013 at 10:25:33AM +0200, Peter Zijlstra wrote:
> On Thu, May 30, 2013 at 02:04:06PM -0400, Johannes Weiner wrote:
> > Previously, page cache radix tree nodes were freed after reclaim
> > emptied out their page pointers. But now reclaim stores shadow
> > entries in their place, which are only reclaimed when the inodes
> > themselves are reclaimed. This is problematic for bigger files that
> > are still in use after they have a significant amount of their cache
> > reclaimed, without any of those pages actually refaulting. The shadow
> > entries will just sit there and waste memory. In the worst case, the
> > shadow entries will accumulate until the machine runs out of memory.
> >
> Can't we simply prune all refault entries that have a distance larger
> than the memory size? Then we must assume that no refault entry means
> its too old, which I think is a fair assumption.

Two workloads bound to two nodes might not push pages through the LRUs
at the same pace, so a distance might be bigger than memory due to the
faster moving node, yet still be a hit in the slower moving one. We
can't really know until we evaluate it on a per-zone basis.
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