linux-kernel - Re: [PATCH v10 3/3] mm: add anonymous vma name refcounting

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Message-ID: <202110051220.FFD4C604EF@keescook>
Date:   Tue, 5 Oct 2021 12:21:26 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Suren Baghdasaryan <surenb@...gle.com>
Cc:     Pavel Machek <pavel@....cz>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Colin Cross <ccross@...gle.com>,
        Sumit Semwal <sumit.semwal@...aro.org>,
        Michal Hocko <mhocko@...e.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Matthew Wilcox <willy@...radead.org>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Johannes Weiner <hannes@...xchg.org>,
        Jonathan Corbet <corbet@....net>,
        Al Viro <viro@...iv.linux.org.uk>,
        Randy Dunlap <rdunlap@...radead.org>,
        Kalesh Singh <kaleshsingh@...gle.com>,
        Peter Xu <peterx@...hat.com>, rppt@...nel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Catalin Marinas <catalin.marinas@....com>,
        vincenzo.frascino@....com,
        Chinwen Chang (張錦文) 
        <chinwen.chang@...iatek.com>,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Jann Horn <jannh@...gle.com>, apopple@...dia.com,
        John Hubbard <jhubbard@...dia.com>,
        Yu Zhao <yuzhao@...gle.com>, Will Deacon <will@...nel.org>,
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        Hugh Dickins <hughd@...gle.com>, feng.tang@...el.com,
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        Jens Axboe <axboe@...nel.dk>, legion@...nel.org,
        Rolf Eike Beer <eb@...ix.com>,
        Cyrill Gorcunov <gorcunov@...il.com>,
        Muchun Song <songmuchun@...edance.com>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Thomas Cedeno <thomascedeno@...gle.com>, sashal@...nel.org,
        cxfcosmos@...il.com, Rasmus Villemoes <linux@...musvillemoes.dk>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-fsdevel@...r.kernel.org, linux-doc@...r.kernel.org,
        linux-mm <linux-mm@...ck.org>,
        kernel-team <kernel-team@...roid.com>
Subject: Re: [PATCH v10 3/3] mm: add anonymous vma name refcounting

On Tue, Oct 05, 2021 at 12:14:59PM -0700, Suren Baghdasaryan wrote:
> On Tue, Oct 5, 2021 at 11:42 AM Pavel Machek <pavel@....cz> wrote:
> >
> > On Fri 2021-10-01 13:56:57, Suren Baghdasaryan wrote:
> > > While forking a process with high number (64K) of named anonymous vmas the
> > > overhead caused by strdup() is noticeable. Experiments with ARM64
> > Android
> >
> > I still believe you should simply use numbers and do the
> > numbers->strings mapping in userspace. We should not need to optimize
> > strdups in kernel...
> 
> Here are complications with mapping numbers to strings in the userspace:
> Approach 1: hardcode number->string in some header file and let all
> tools use that mapping. The issue is that whenever that mapping
> changes all the tools that are using it (including 3rd party ones)
> have to be rebuilt. This is not really maintainable since we don't
> control 3rd party tools and even for the ones we control, it will be a
> maintenance issue figuring out which version of the tool used which
> header file.
> Approach 2: have a centralized facility (a process or a DB)
> maintaining number->string mapping. This would require an additional
> request to this facility whenever we want to make a number->string
> conversion. Moreover, when we want to name a VMA, we would have to
> register a new VMA name in that facility or check that one already
> exists and get its ID. So each prctl() call to name a VMA will be
> preceded by such a request (IPC call), maybe with some optimizations
> to cache already known number->string pairs. This would be quite
> expensive performance-wise. Additional issue with this approach is
> that this mapping will have to be persistent to handle a case when the
> facility crashes and has to be restored.
> 
> As I said before, it complicates userspace quite a bit. Is that a good
> enough reason to store the names in the kernel and pay a little more
> memory for that? IMHO yes, but I might be wrong.

FWIW, I prefer the strings. It's more human-readable, which is important
for the kinds of cases where the maps are being used for diagnostics,
etc.

-- 
Kees Cook