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 11:59:03 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     Mike Rapoport <rppt@...nel.org>
Cc:     David Hildenbrand <david@...hat.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>,
        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 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?
-- 
Michal Hocko
SUSE Labs