linux-kernel - Re: [PATCH v16 07/11] secretmem: use PMD-size pages to amortize direct map fragmentation

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Date:   Tue, 2 Feb 2021 15:12:21 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Michal Hocko <mhocko@...e.com>
Cc:     Mike Rapoport <rppt@...nel.org>,
        James Bottomley <jejb@...ux.ibm.com>,
        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>,
        "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, Hagen Paul Pfeifer <hagen@...u.net>,
        Palmer Dabbelt <palmerdabbelt@...gle.com>
Subject: Re: [PATCH v16 07/11] secretmem: use PMD-size pages to amortize
 direct map fragmentation

On 02.02.21 14:32, Michal Hocko wrote:
> 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).

Yeah, however that is a very special case. In most cases mlock() simply 
prevents swapping, you still have movable pages you can place anywhere 
you like (including on ZONE_MOVABLE).

> 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.

Right, but again, special cases - e.g., tmpfs explicitly has to be resized.

> If secretmem behaves along those existing models
> then we know what to expect at least.

I think secretmem behaves much more like longterm GUP right now 
("unmigratable", "lifetime controlled by user space", "cannot go on 
CMA/ZONE_MOVABLE"). I'd either want to reasonably well control/limit it 
or make it behave more like mlocked pages.

-- 
Thanks,

David / dhildenb