Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Date:   Fri, 25 Sep 2020 11:50:29 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     David Hildenbrand <david@redhat.com>
Cc:     Mike Rapoport <rppt@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Andy Lutomirski <luto@kernel.org>,
        Arnd Bergmann <arnd@arndb.de>, Borislav Petkov <bp@alien8.de>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Christopher Lameter <cl@linux.com>,
        Dan Williams <dan.j.williams@intel.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Elena Reshetova <elena.reshetova@intel.com>,
        "H. Peter Anvin" <hpa@zytor.com>, Idan Yaniv <idan.yaniv@ibm.com>,
        Ingo Molnar <mingo@redhat.com>,
        James Bottomley <jejb@linux.ibm.com>,
        "Kirill A. Shutemov" <kirill@shutemov.name>,
        Matthew Wilcox <willy@infradead.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Mike Rapoport <rppt@linux.ibm.com>,
        Michael Kerrisk <mtk.manpages@gmail.com>,
        Palmer Dabbelt <palmer@dabbelt.com>,
        Paul Walmsley <paul.walmsley@sifive.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Shuah Khan <shuah@kernel.org>, Tycho Andersen <tycho@tycho.ws>,
        Will Deacon <will@kernel.org>, linux-api@vger.kernel.org,
        linux-arch@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
        linux-fsdevel@vger.kernel.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org,
        linux-nvdimm@lists.01.org, linux-riscv@lists.infradead.org,
        x86@kernel.org
Subject: Re: [PATCH v6 5/6] mm: secretmem: use PMD-size pages to amortize
 direct map fragmentation
Message-ID: <20200925095029.GX2628@hirez.programming.kicks-ass.net>
References: <20200924132904.1391-1-rppt@kernel.org>
 <20200924132904.1391-6-rppt@kernel.org>
 <20200925074125.GQ2628@hirez.programming.kicks-ass.net>
 <8435eff6-7fa9-d923-45e5-d8850e4c6d73@redhat.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <8435eff6-7fa9-d923-45e5-d8850e4c6d73@redhat.com>
Precedence: bulk

On Fri, Sep 25, 2020 at 11:00:30AM +0200, David Hildenbrand wrote:
> On 25.09.20 09:41, Peter Zijlstra wrote:
> > On Thu, Sep 24, 2020 at 04:29:03PM +0300, Mike Rapoport wrote:
> >> From: Mike Rapoport <rppt@linux.ibm.com>
> >>
> >> Removing a PAGE_SIZE page from the direct map every time such page is
> >> allocated for a secret memory mapping will cause severe fragmentation of
> >> the direct map. This fragmentation can be reduced by using PMD-size pages
> >> as a pool for small pages for secret memory mappings.
> >>
> >> Add a gen_pool per secretmem inode and lazily populate this pool with
> >> PMD-size pages.
> > 
> > What's the actual efficacy of this? Since the pmd is per inode, all I
> > need is a lot of inodes and we're in business to destroy the directmap,
> > no?
> > 
> > Afaict there's no privs needed to use this, all a process needs is to
> > stay below the mlock limit, so a 'fork-bomb' that maps a single secret
> > page will utterly destroy the direct map.
> > 
> > I really don't like this, at all.
> 
> As I expressed earlier, I would prefer allowing allocation of secretmem
> only from a previously defined CMA area. This would physically locally
> limit the pain.

Given that this thing doesn't have a migrate hook, that seems like an
eminently reasonable contraint. Because not only will it mess up the
directmap, it will also destroy the ability of the page-allocator /
compaction to re-form high order blocks by sprinkling holes throughout.

Also, this is all very close to XPFO, yet I don't see that mentioned
anywhere.

Further still, it has this HAVE_SECRETMEM_UNCACHED nonsense which is
completely unused. I'm not at all sure exposing UNCACHED to random
userspace is a sane idea.