linux-kernel - Re: [PATCH v6 5/6] mm: secretmem: use PMD-size pages to amortize direct map fragmentation

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Message-ID: <20200930150928.GR20115@casper.infradead.org>
Date:   Wed, 30 Sep 2020 16:09:28 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Mike Rapoport <rppt@...ux.ibm.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Mike Rapoport <rppt@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Andy Lutomirski <luto@...nel.org>,
        Arnd Bergmann <arnd@...db.de>, Borislav Petkov <bp@...en8.de>,
        Catalin Marinas <catalin.marinas@....com>,
        Christopher Lameter <cl@...ux.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        David Hildenbrand <david@...hat.com>,
        Elena Reshetova <elena.reshetova@...el.com>,
        "H. Peter Anvin" <hpa@...or.com>, Idan Yaniv <idan.yaniv@....com>,
        Ingo Molnar <mingo@...hat.com>,
        James Bottomley <jejb@...ux.ibm.com>,
        "Kirill A. Shutemov" <kirill@...temov.name>,
        Mark Rutland <mark.rutland@....com>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Shuah Khan <shuah@...nel.org>, Tycho Andersen <tycho@...ho.ws>,
        Will Deacon <will@...nel.org>, linux-api@...r.kernel.org,
        linux-arch@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org,
        linux-nvdimm@...ts.01.org, linux-riscv@...ts.infradead.org,
        x86@...nel.org
Subject: Re: [PATCH v6 5/6] mm: secretmem: use PMD-size pages to amortize
 direct map fragmentation

On Wed, Sep 30, 2020 at 01:27:45PM +0300, Mike Rapoport wrote:
> On Tue, Sep 29, 2020 at 05:15:52PM +0200, Peter Zijlstra wrote:
> > On Tue, Sep 29, 2020 at 05:58:13PM +0300, Mike Rapoport wrote:
> > > On Tue, Sep 29, 2020 at 04:12:16PM +0200, Peter Zijlstra wrote:
> > 
> > > > It will drop them down to 4k pages. Given enough inodes, and allocating
> > > > only a single sekrit page per pmd, we'll shatter the directmap into 4k.
> > > 
> > > Why? Secretmem allocates PMD-size page per inode and uses it as a pool
> > > of 4K pages for that inode. This way it ensures that
> > > __kernel_map_pages() is always called on PMD boundaries.
> > 
> > Oh, you unmap the 2m page upfront? I read it like you did the unmap at
> > the sekrit page alloc, not the pool alloc side of things.
> > 
> > Then yes, but then you're wasting gobs of memory. Basically you can pin
> > 2M per inode while only accounting a single page.
> 
> Right, quite like THP :)

Huh?  THP accounts every page it allocates.  If you allocate 2MB,
it accounts 512 pages.  And THP are reclaimable by vmscan, this is
obviously not.