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Date:   Wed, 7 Apr 2021 03:35:27 +0100
From:   Matthew Wilcox <willy@infradead.org>
To:     Michel Lespinasse <michel@lespinasse.org>
Cc:     Linux-MM <linux-mm@kvack.org>,
        Laurent Dufour <ldufour@linux.ibm.com>,
        Peter Zijlstra <peterz@infradead.org>,
        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()
Message-ID: <20210407023527.GX2531743@casper.infradead.org>
References: <20210407014502.24091-1-michel@lespinasse.org>
 <20210407014502.24091-25-michel@lespinasse.org>
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On Tue, Apr 06, 2021 at 06:44:49PM -0700, Michel Lespinasse wrote:
> In the speculative case, call the vm_ops->fault() method from within
> an rcu read locked section, and verify the mmap sequence lock at the
> start of the section. A match guarantees that the original vma is still
> valid at that time, and that the associated vma->vm_file stays valid
> while the vm_ops->fault() method is running.
> 
> Note that this implies that speculative faults can not sleep within
> the vm_ops->fault method. We will only attempt to fetch existing pages
> from the page cache during speculative faults; any miss (or prefetch)
> will be handled by falling back to non-speculative fault handling.
> 
> The speculative handling case also does not preallocate page tables,
> as it is always called with a pre-existing page table.

I still don't understand why you want to do this.  The speculative
fault that doesn't do I/O is already here, and it's called ->map_pages
(which I see you also do later).  So what's the point of this patch?