linux-kernel - Re: [RFC PATCH 24/37] mm: implement speculative handling in __do

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Message-ID: <YG6qCtRcz2ESUiFy@hirez.programming.kicks-ass.net>
Date:   Thu, 8 Apr 2021 09:00:26 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Michel Lespinasse <michel@...pinasse.org>,
        Linux-MM <linux-mm@...ck.org>,
        Laurent Dufour <ldufour@...ux.ibm.com>,
        Michal Hocko <mhocko@...e.com>,
        Rik van Riel <riel@...riel.com>,
        Paul McKenney <paulmck@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Joel Fernandes <joelaf@...gle.com>,
        Rom Lemarchand <romlem@...gle.com>,
        Linux-Kernel <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH 24/37] mm: implement speculative handling in
 __do_fault()

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 :/