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Date:   Wed, 11 Jul 2018 16:10:30 -0700
From:   Andrew Morton <akpm@linux-foundation.org>
To:     David Rientjes <rientjes@google.com>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        Davidlohr Bueso <dave@stgolabs.net>,
        Alexey Dobriyan <adobriyan@gmail.com>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org
Subject: Re: [patch] mm, vmacache: hash addresses based on pmd
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On Mon, 9 Jul 2018 18:37:37 -0700 (PDT) David Rientjes <rientjes@google.com> wrote:

> > Did you consider LRU-sorting the array instead?
> > 
> 
> It adds 40 bytes to struct task_struct,

What does?  LRU sort?  It's a 4-entry array, just do it in place, like
bh_lru_install(). Confused.

> but I'm not sure the least 
> recently used is the first preferred check.  If I do 
> madvise(MADV_DONTNEED) from a malloc implementation where I don't control 
> what is free()'d and I'm constantly freeing back to the same hugepages, 
> for example, I may always get first slot cache hits with this patch as 
> opposed to the 25% chance that the current implementation has (and perhaps 
> an lru would as well).
> 
> I'm sure that I could construct a workload where LRU would be better and 
> could show that the added footprint were worthwhile, but I could also 
> construct a workload where the current implementation based on pfn would 
> outperform all of these.  It simply turns out that on the user-controlled 
> workloads that I was profiling that hashing based on pmd was the win.

That leaves us nowhere to go.  Zapping the WARN_ON seems a no-brainer
though?