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From:   "Zach O'Keefe" <zokeefe@google.com>
Date:   Tue, 2 Aug 2022 12:42:48 -0700
Message-ID: <CAAa6QmS2aAtJyWPFi1to99o=vHWFmiQRW72+3HdZ-4qGk2FT5Q@mail.gmail.com>
Subject: Re: [PATCH mm-unstable] mm/madvise: remove CAP_SYS_ADMIN requirement
 for process_madvise(MADV_COLLAPSE)
To:     Michal Hocko <mhocko@suse.com>
Cc:     linux-mm@kvack.org, Andrew Morton <akpm@linux-foundation.org>,
        linux-api@vger.kernel.org, linux-kernel@vger.kernel.org,
        Axel Rasmussen <axelrasmussen@google.com>,
        James Houghton <jthoughton@google.com>,
        Hugh Dickins <hughd@google.com>,
        Yang Shi <shy828301@gmail.com>,
        Miaohe Lin <linmiaohe@huawei.com>,
        David Hildenbrand <david@redhat.com>,
        David Rientjes <rientjes@google.com>,
        Matthew Wilcox <willy@infradead.org>,
        Pasha Tatashin <pasha.tatashin@soleen.com>,
        Peter Xu <peterx@redhat.com>,
        Rongwei Wang <rongwei.wang@linux.alibaba.com>,
        SeongJae Park <sj@kernel.org>,
        Song Liu <songliubraving@fb.com>,
        Vlastimil Babka <vbabka@suse.cz>, Zi Yan <ziy@nvidia.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Arnd Bergmann <arnd@arndb.de>,
        Chris Kennelly <ckennelly@google.com>,
        Chris Zankel <chris@zankel.net>, Helge Deller <deller@gmx.de>,
        Ivan Kokshaysky <ink@jurassic.park.msu.ru>,
        "James E.J. Bottomley" <James.Bottomley@hansenpartnership.com>,
        Jens Axboe <axboe@kernel.dk>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Matt Turner <mattst88@gmail.com>,
        Max Filippov <jcmvbkbc@gmail.com>,
        Minchan Kim <minchan@kernel.org>,
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        Pavel Begunkov <asml.silence@gmail.com>,
        Thomas Bogendoerfer <tsbogend@alpha.franken.de>
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On Tue, Aug 2, 2022 at 5:04 AM Michal Hocko <mhocko@suse.com> wrote:
>
> On Tue 02-08-22 02:48:33, Zach O'Keefe wrote:
> [...]
> > "mm/madvise: add MADV_COLLAPSE to process_madvise()" in the v7 series
> > ended with me mentioning a couple options, but ultimately I didn't
> > present a solution, and no consensus was reached[1]. After taking a
> > closer look, this is my proposal for what I believe to be the best
> > path forward. It should be squashed into the original patch. What do you think?
>
> If it is agreed that the CAP_SYS_ADMIN is too strict of a requirement
> then yes, this should be squashed into the original patch. There is no
> real reason to create a potential bisection headache by changing the
> permission model in a later patch.

Sorry about the confusion here. Assumed (incorrectly) that Andrew
would kindly squash this in mm-unstable since I added the Fixes: tag.
Next time I'll add some explicit verbiage saying it should be
squashed.

> From my POV, I would agree that CAP_SYS_ADMIN is just too strict of a
> requirement.
>
> I didn't really have time to follow recent discussions but I would argue
> that the operation is not really destructive or seriously harmful. All
> applications can already have their memory (almost) equally THP
> collapsed by khupaged with the proposed process_madvise semantic.
>
> NOHUGEMEM and prctl opt out from THP are both honored AFAIU and the only
> difference is the global THP killswitch behavior which I do not think
> warrants the strongest CAP_SYS_ADMIN capability (especially because it
> doesn't really control all kinds of THPs).

Ya. In fact, I don't think the ignoring the THP sysfs controls
warrants any additional capability (set alone CAPS_SYS_ADMIN), since a
malicious program can't really inflict any more damage than they would
with CAP_SYS_NICE and PTRACE_MODE_READ.

> If there is a userspace agent collapsing memory and causing problems
> then it can be easily fixed in the userspace. And I find that easier
> to do than putting the bar so high that userspace agents would be
> unfeasible because of CAP_SYS_ADMIN (which is nono in many cases as it
> would allow essentially full control of other stuff). So from practical
> POV, risking an extended RSS is really a negligible risk to lose a
> potentially useful feature for all others.
>

Agreed.

Thanks for taking the time, Michal!
Zach


> Just my 2c
>
> > Thanks again,
> > Zach
> >
> > [1] https://lore.kernel.org/linux-mm/Ys4aTRqWIbjNs1mI@google.com/
>
> --
> Michal Hocko
> SUSE Labs