linux-kernel - Re: [PATCH v19 5/8] mm: introduce memfd_secret system call to create "secret" memory areas

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Date:   Tue, 18 May 2021 12:06:42 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Michal Hocko <mhocko@...e.com>, Mike Rapoport <rppt@...nel.org>
Cc:     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>,
        Hagen Paul Pfeifer <hagen@...u.net>,
        Ingo Molnar <mingo@...hat.com>,
        James Bottomley <jejb@...ux.ibm.com>,
        Kees Cook <keescook@...omium.org>,
        "Kirill A. Shutemov" <kirill@...temov.name>,
        Matthew Wilcox <willy@...radead.org>,
        Matthew Garrett <mjg59@...f.ucam.org>,
        Mark Rutland <mark.rutland@....com>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Palmer Dabbelt <palmerdabbelt@...gle.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Peter Zijlstra <peterz@...radead.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        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>,
        Yury Norov <yury.norov@...il.com>, 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 v19 5/8] mm: introduce memfd_secret system call to create
 "secret" memory areas

On 18.05.21 11:59, Michal Hocko wrote:
> On Sun 16-05-21 10:29:24, Mike Rapoport wrote:
>> On Fri, May 14, 2021 at 11:25:43AM +0200, David Hildenbrand wrote:
> [...]
>>>> +		if (!page)
>>>> +			return VM_FAULT_OOM;
>>>> +
>>>> +		err = set_direct_map_invalid_noflush(page, 1);
>>>> +		if (err) {
>>>> +			put_page(page);
>>>> +			return vmf_error(err);
>>>
>>> Would we want to translate that to a proper VM_FAULT_..., which would most
>>> probably be VM_FAULT_OOM when we fail to allocate a pagetable?
>>
>> That's what vmf_error does, it translates -ESOMETHING to VM_FAULT_XYZ.
> 
> I haven't read through the rest but this has just caught my attention.
> Is it really reasonable to trigger the oom killer when you cannot
> invalidate the direct mapping. From a quick look at the code it is quite
> unlikely to se ENOMEM from that path (it allocates small pages) but this
> can become quite sublte over time. Shouldn't this simply SIGBUS if it
> cannot manipulate the direct mapping regardless of the underlying reason
> for that?
> 

OTOH, it means our kernel zones are depleted, so we'd better reclaim 
somehow ...

-- 
Thanks,

David / dhildenb