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

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Message-ID: <CAJuCfpGTCM_Rf3GEyzpR5UOTfgGKTY0_rvAbGdtjbyabFhrRAw@mail.gmail.com>
Date:   Tue, 12 Oct 2021 10:01:05 -0700
From:   Suren Baghdasaryan <surenb@...gle.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     Michal Hocko <mhocko@...e.com>, Kees Cook <keescook@...omium.org>,
        Pavel Machek <pavel@....cz>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        John Hubbard <jhubbard@...dia.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
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        Sumit Semwal <sumit.semwal@...aro.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Matthew Wilcox <willy@...radead.org>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
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        Johannes Weiner <hannes@...xchg.org>,
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        Al Viro <viro@...iv.linux.org.uk>,
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Subject: Re: [PATCH v10 3/3] mm: add anonymous vma name refcounting

On Tue, Oct 12, 2021 at 12:44 AM David Hildenbrand <david@...hat.com> wrote:
>
> > I'm still evaluating the proposal to use memfds but I'm not sure if
> > the issue that David Hildenbrand mentioned about additional memory
> > consumed in pagecache (which has to be addressed) is the only one we
> > will encounter with this approach. If anyone knows of any potential
> > issues with using memfds as named anonymous memory, I would really
> > appreciate your feedback before I go too far in that direction.
>
> [MAP_PRIVATE memfd only behave that way with 4k, not with huge pages, so
> I think it just has to be fixed. It doesn't make any sense to allocate a
> page for the pagecache ("populate the file") when accessing via a
> private mapping that's supposed to leave the file untouched]
>
> My gut feeling is if you really need a string as identifier, then try
> going with memfds. Yes, we might hit some road blocks to be sorted out,
> but it just logically makes sense to me: Files have names. These names
> exist before mapping and after mapping. They "name" the content.

I'm investigating this direction. I don't have much background with
memfds, so I'll need to digest the code first.

>
> Maybe it's just me, but the whole interface, setting the name via a
> prctl after the mapping was already instantiated doesn't really spark
> joy at my end. That's not a strong pushback, but if we can avoid it
> using something that's already there, that would be very much preferred.

Actually that's one of my worries about using memfds. There might be
cases when we need to name a vma after it was mapped. memfd_create()
would not allow us to do that AFAIKT. But I need to check all usages
to say if that's really an issue.
Thanks!

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