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Date:   Thu, 8 Apr 2021 09:00:26 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Matthew Wilcox <willy@infradead.org>
Cc:     Michel Lespinasse <michel@lespinasse.org>,
        Linux-MM <linux-mm@kvack.org>,
        Laurent Dufour <ldufour@linux.ibm.com>,
        Michal Hocko <mhocko@suse.com>,
        Rik van Riel <riel@surriel.com>,
        Paul McKenney <paulmck@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Suren Baghdasaryan <surenb@google.com>,
        Joel Fernandes <joelaf@google.com>,
        Rom Lemarchand <romlem@google.com>,
        Linux-Kernel <linux-kernel@vger.kernel.org>
Subject: Re: [RFC PATCH 24/37] mm: implement speculative handling in
 __do_fault()
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On Wed, Apr 07, 2021 at 10:27:12PM +0100, Matthew Wilcox wrote:
> Doing I/O without any lock held already works; it just uses the file
> refcount.  It would be better to use a vma refcount, as I already said.

The original workload that I developed SPF for (waaaay back when) was
prefaulting a single huge vma. Using a vma refcount was a total loss
because it resulted in the same cacheline contention that down_read()
was having.

As such, I'm always incredibly sad to see mention of vma refcounts.
They're fundamentally not solving the problem :/