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Date:   Mon, 11 May 2020 19:02:12 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Matthew Wilcox <willy@infradead.org>
Cc:     Joerg Roedel <jroedel@suse.de>, Joerg Roedel <joro@8bytes.org>,
        x86@kernel.org, hpa@zytor.com,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Andy Lutomirski <luto@kernel.org>, rjw@rjwysocki.net,
        Arnd Bergmann <arnd@arndb.de>,
        Andrew Morton <akpm@linux-foundation.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Vlastimil Babka <vbabka@suse.cz>,
        Michal Hocko <mhocko@kernel.org>, linux-kernel@vger.kernel.org,
        linux-acpi@vger.kernel.org, linux-arch@vger.kernel.org,
        linux-mm@kvack.org
Subject: Re: [RFC PATCH 0/7] mm: Get rid of vmalloc_sync_(un)mappings()
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On Mon, May 11, 2020 at 08:52:04AM -0700, Matthew Wilcox wrote:
> On Mon, May 11, 2020 at 09:31:34AM +0200, Peter Zijlstra wrote:
> > On Sat, May 09, 2020 at 06:11:57PM -0700, Matthew Wilcox wrote:
> > > Iterating an XArray (whether the entire thing
> > > or with marks) is RCU-safe and faster than iterating a linked list,
> > > so this should solve the problem?
> > 
> > It can hardly be faster if you want all elements -- which is I think the
> > case here. We only call into this if we change an entry, and then we
> > need to propagate that change to all.
> 
> Of course it can be faster.  Iterating an array is faster than iterating
> a linked list because caches.  While an XArray is a segmented array
> (so slower than a plain array), it's plainly going to be faster than
> iterating a linked list.

Fair enough, mostly also because the actual work (setting a single PTE)
doesn't dominate in this case.