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Date:   Tue, 29 Sep 2020 16:06:41 +0300
From:   Mike Rapoport <rppt@kernel.org>
To:     David Hildenbrand <david@redhat.com>
Cc:     Peter Zijlstra <peterz@infradead.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>,
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        Thomas Gleixner <tglx@linutronix.de>,
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        x86@kernel.org
Subject: Re: [PATCH v6 5/6] mm: secretmem: use PMD-size pages to amortize
 direct map fragmentation
Message-ID: <20200929130641.GG2142832@kernel.org>
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>
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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.

The prevois version contained a patch that allowed reserving a memory
pool for the secretmem at boot time to avpoid splitting pages from the
direct map

> But my suggestion was not well received :)

The disagreemet was only whether to use CMA or simple boot time
reservation :-P

> -- 
> Thanks,
> 
> David / dhildenb
> 

-- 
Sincerely yours,
Mike.