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

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Message-ID: <20210407023527.GX2531743@casper.infradead.org>
Date:   Wed, 7 Apr 2021 03:35:27 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Michel Lespinasse <michel@...pinasse.org>
Cc:     Linux-MM <linux-mm@...ck.org>,
        Laurent Dufour <ldufour@...ux.ibm.com>,
        Peter Zijlstra <peterz@...radead.org>,
        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 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?