linux-kernel - Re: [RFC PATCH 0/7] mm: Get rid of vmalloc_sync

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Date:   Mon, 11 May 2020 19:06:46 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Joerg Roedel <jroedel@...e.de>, Joerg Roedel <joro@...tes.org>,
        X86 ML <x86@...nel.org>, "H. Peter Anvin" <hpa@...or.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Arnd Bergmann <arnd@...db.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Michal Hocko <mhocko@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux ACPI <linux-acpi@...r.kernel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>
Subject: Re: [RFC PATCH 0/7] mm: Get rid of vmalloc_sync_(un)mappings()

On Mon, May 11, 2020 at 08:36:31AM -0700, Andy Lutomirski wrote:
> On Mon, May 11, 2020 at 12:42 AM Peter Zijlstra <peterz@...radead.org> wrote:
> >
> > On Sat, May 09, 2020 at 12:05:29PM -0700, Andy Lutomirski wrote:
> >
> > > On x86_64, the only real advantage is that the handful of corner cases
> > > that make vmalloc faults unpleasant (mostly relating to vmap stacks)
> > > go away.  On x86_32, a bunch of mind-bending stuff (everything your
> > > series deletes but also almost everything your series *adds*) goes
> > > away.  There may be a genuine tiny performance hit on 2-level systems
> > > due to the loss of huge pages in vmalloc space, but I'm not sure I
> > > care or that we use them anyway on these systems.  And PeterZ can stop
> > > even thinking about RCU.
> > >
> > > Am I making sense?
> >
> > I think it'll work for x86_64 and that is really all I care about :-)
> 
> Sadly, I think that Joerg has convinced my that this doesn't really
> work for 32-bit unless we rework the LDT code or drop support for
> something that we might not want to drop support for.

I was thinking keep these patches for 32bit and fix 64bit 'proper'. But
sure, if we can get rid of it all by stripping 32bit features I'm not
going to object either.