Date:   Thu, 7 Oct 2021 08:42:20 -0700
From:   Suren Baghdasaryan <surenb@...gle.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     Michal Hocko <mhocko@...e.com>, John Hubbard <jhubbard@...dia.com>,
        Pavel Machek <pavel@....cz>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Colin Cross <ccross@...gle.com>,
        Sumit Semwal <sumit.semwal@...aro.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Kees Cook <keescook@...omium.org>,
        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,
        Yu Zhao <yuzhao@...gle.com>, Will Deacon <will@...nel.org>,
        fenghua.yu@...el.com, thunder.leizhen@...wei.com,
        Hugh Dickins <hughd@...gle.com>, feng.tang@...el.com,
        Jason Gunthorpe <jgg@...pe.ca>, Roman Gushchin <guro@...com>,
        Thomas Gleixner <tglx@...utronix.de>, krisman@...labora.com,
        chris.hyser@...cle.com, Peter Collingbourne <pcc@...gle.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        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 Thu, Oct 7, 2021 at 12:33 AM David Hildenbrand <david@...hat.com> wrote:
>
> On 06.10.21 17:20, Suren Baghdasaryan wrote:
> > On Wed, Oct 6, 2021 at 8:08 AM David Hildenbrand <david@...hat.com> wrote:
> >>
> >> On 06.10.21 17:01, Suren Baghdasaryan wrote:
> >>> On Wed, Oct 6, 2021 at 2:27 AM David Hildenbrand <david@...hat.com> wrote:
> >>>>
> >>>> On 06.10.21 10:27, Michal Hocko wrote:
> >>>>> On Tue 05-10-21 23:57:36, John Hubbard wrote:
> >>>>> [...]
> >>>>>> 1) Yes, just leave the strings in the kernel, that's simple and
> >>>>>> it works, and the alternatives don't really help your case nearly
> >>>>>> enough.
> >>>>>
> >>>>> I do not have a strong opinion. Strings are easier to use but they
> >>>>> are more involved and the necessity of kref approach just underlines
> >>>>> that. There are going to be new allocations and that always can lead
> >>>>> to surprising side effects.  These are small (80B at maximum) so the
> >>>>> overall footpring shouldn't all that large by default but it can grow
> >>>>> quite large with a very high max_map_count. There are workloads which
> >>>>> really require the default to be set high (e.g. heavy mremap users). So
> >>>>> if anything all those should be __GFP_ACCOUNT and memcg accounted.
> >>>>>
> >>>>> I do agree that numbers are just much more simpler from accounting,
> >>>>> performance and implementation POV.
> >>>>
> >>>> +1
> >>>>
> >>>> I can understand that having a string can be quite beneficial e.g., when
> >>>> dumping mmaps. If only user space knows the id <-> string mapping, that
> >>>> can be quite tricky.
> >>>>
> >>>> However, I also do wonder if there would be a way to standardize/reserve
> >>>> ids, such that a given id always corresponds to a specific user. If we
> >>>> use an uint64_t for an id, there would be plenty room to reserve ids ...
> >>>>
> >>>> I'd really prefer if we can avoid using strings and instead using ids.
> >>>
> >>> I wish it was that simple and for some names like [anon:.bss] or
> >>> [anon:dalvik-zygote space] reserving a unique id would work, however
> >>> some names like [anon:dalvik-/system/framework/boot-core-icu4j.art]
> >>> are generated dynamically at runtime and include package name.
> >>
> >> Valuable information
> >
> > Yeah, I should have described it clearer the first time around.
> >
> >>
> >>> Packages are constantly evolving, new ones are developed, names can
> >>> change, etc. So assigning a unique id for these names is not really
> >>> feasible.
> >>
> >> So, you'd actually want to generate/reserve an id for a given string at
> >> runtime, assign that id to the VMA, and have a way to match id <->
> >> string somehow?
> >
> > If we go with ids then yes, that is what we would have to do.
> >
> >> That reservation service could be inside the kernel or even (better?) in
> >> user space. The service could for example de-duplicates strings.
> >
> > Yes but it would require an IPC call to that service potentially on
> > every mmap() when we want to name a mapped vma. This would be
> > prohibitive. Even on consumption side, instead of just dumping
> > /proc/$pid/maps we would have to parse the file and convert all
> > [anon:id] into [anon:name] with each conversion requiring an IPC call
> > (assuming no id->name pair caching on the client side).
>
> mmap() and prctl() already do take the mmap sem in write, so they are
> not the "most lightweight" operations so to say.
>
> We already to have two separate operations, first the mmap(), then the
> prctl(). IMHO you could defer the "naming" part to a later point in
> time, without creating too many issues, moving it out of the
> "hot/performance critical phase"
>
> Reading https://lwn.net/Articles/867818/, to me it feels like the use
> case could live with a little larger delay between the mmap popping up
> and a name getting assigned.

That might be doable if occasional inconsistency can be tolerated (we
can't guarantee that maps won't be read before the deferred work name
the vma). However I would prefer an efficient solution vs the one
which is inefficient but can be deferred.

>
> --
> Thanks,
>
> David / dhildenb
>

Powered by blists - more mailing lists