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Date:   Tue, 2 Feb 2021 14:32:13 +0100
From:   Michal Hocko <mhocko@suse.com>
To:     David Hildenbrand <david@redhat.com>
Cc:     Mike Rapoport <rppt@kernel.org>,
        James Bottomley <jejb@linux.ibm.com>,
        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>,
        "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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        Shakeel Butt <shakeelb@google.com>,
        Shuah Khan <shuah@kernel.org>,
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        Palmer Dabbelt <palmerdabbelt@google.com>
Subject: Re: [PATCH v16 07/11] secretmem: use PMD-size pages to amortize
 direct map fragmentation
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On Tue 02-02-21 14:14:09, David Hildenbrand wrote:
[...]
> As already expressed, I dislike allowing user space to consume an unlimited
> number unmovable/unmigratable allocations. We already have that in some
> cases with huge pages (when the arch does not support migration) - but there
> we can at least manage the consumption using the whole max/reserved/free/...
> infrastructure. In addition, adding arch support for migration shouldn't be
> too complicated.

Well, mlock is not too different here as well. Hugepages are arguably an
easier model because it requires an explicit pre-configuration by an
admin. Mlock doesn't have anything like that. Please also note that
while mlock pages are migrateable by default, this is not the case in
general because they can be configured to disalow migration to prevent
from minor page faults as some workloads require that (e.g. RT).
Another example is ramdisk or even tmpfs (with swap storage depleted or
not configured). Both are PITA from the OOM POV but they are manageable
if people are careful. If secretmem behaves along those existing models
then we know what to expect at least.
-- 
Michal Hocko
SUSE Labs