linux-kernel - Re: [PATCH v17 00/10] mm: introduce memfd_secret system call to create "secret" memory areas

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Message-ID: <YCJO8zLq8YkXGy8B@dhcp22.suse.cz>
Date:   Tue, 9 Feb 2021 09:59:31 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     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>,
        Elena Reshetova <elena.reshetova@...el.com>,
        "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
        James Bottomley <jejb@...ux.ibm.com>,
        "Kirill A. Shutemov" <kirill@...temov.name>,
        Matthew Wilcox <willy@...radead.org>,
        Mark Rutland <mark.rutland@....com>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Rick Edgecombe <rick.p.edgecombe@...el.com>,
        Roman Gushchin <guro@...com>,
        Shakeel Butt <shakeelb@...gle.com>,
        Shuah Khan <shuah@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        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 v17 00/10] mm: introduce memfd_secret system call to
 create "secret" memory areas

On Mon 08-02-21 22:38:03, David Hildenbrand wrote:
> 
> > Am 08.02.2021 um 22:13 schrieb Mike Rapoport <rppt@...nel.org>:
> > 
> > On Mon, Feb 08, 2021 at 10:27:18AM +0100, David Hildenbrand wrote:
> >> On 08.02.21 09:49, Mike Rapoport wrote:
> >> 
> >> Some questions (and request to document the answers) as we now allow to have
> >> unmovable allocations all over the place and I don't see a single comment
> >> regarding that in the cover letter:
> >> 
> >> 1. How will the issue of plenty of unmovable allocations for user space be
> >> tackled in the future?
> >> 
> >> 2. How has this issue been documented? E.g., interaction with ZONE_MOVABLE
> >> and CMA, alloc_conig_range()/alloc_contig_pages?.
> > 
> > Secretmem sets the mappings gfp mask to GFP_HIGHUSER, so it does not
> > allocate movable pages at the first place.
> 
> That is not the point. Secretmem cannot go on CMA / ZONE_MOVABLE
> memory and behaves like long-term pinnings in that sense. This is a
> real issue when using a lot of sectremem.

A lot of unevictable memory is a concern regardless of CMA/ZONE_MOVABLE.
As I've said it is quite easy to land at the similar situation even with
tmpfs/MAP_ANON|MAP_SHARED on swapless system. Neither of the two is
really uncommon. It would be even worse that those would be allowed to
consume both CMA/ZONE_MOVABLE.

One has to be very careful when relying on CMA or movable zones. This is
definitely worth a comment in the kernel command line parameter
documentation. But this is not a new problem.

-- 
Michal Hocko
SUSE Labs