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Date:   Tue, 9 Feb 2021 09:59:31 +0100
From:   Michal Hocko <mhocko@suse.com>
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>, 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>,
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Subject: Re: [PATCH v17 00/10] mm: introduce memfd_secret system call to
 create "secret" memory areas
Message-ID: <YCJO8zLq8YkXGy8B@dhcp22.suse.cz>
References: <20210208211326.GV242749@kernel.org>
 <1F6A73CF-158A-4261-AA6C-1F5C77F4F326@redhat.com>
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On Mon 08-02-21 22:38:03, David Hildenbrand wrote:
> 
> > Am 08.02.2021 um 22:13 schrieb Mike Rapoport <rppt@kernel.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