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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Wed, 24 Feb 2021 11:10:43 -0800
Message-ID: <CAHk-=wh3wVpx97e=n2D98W+PSDWUkQrX3O+c7n7MGRbn_k9JMg@mail.gmail.com>
Subject: Re: d28296d248: stress-ng.sigsegv.ops_per_sec -82.7% regression
To:     Alexey Gladkov <gladkov.alexey@gmail.com>
Cc:     "Eric W. Biederman" <ebiederm@xmission.com>,
        kernel test robot <oliver.sang@intel.com>,
        0day robot <lkp@intel.com>,
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        "Huang, Ying" <ying.huang@intel.com>,
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        io-uring <io-uring@vger.kernel.org>,
        Kernel Hardening <kernel-hardening@lists.openwall.com>,
        Linux Containers <containers@lists.linux-foundation.org>,
        Linux-MM <linux-mm@kvack.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Christian Brauner <christian.brauner@ubuntu.com>,
        Jann Horn <jannh@google.com>, Jens Axboe <axboe@kernel.dk>,
        Kees Cook <keescook@chromium.org>,
        Oleg Nesterov <oleg@redhat.com>
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On Wed, Feb 24, 2021 at 10:38 AM Alexey Gladkov
<gladkov.alexey@gmail.com> wrote:
>
> One of the reasons for this is that I rolled back the patch that changed
> the ucounts.count type to atomic_t. Now get_ucounts() is forced to use a
> spin_lock to increase the reference count.

Yeah, that definitely should be an atomic type, since the extended use
of ucounts clearly puts way too much pressure on that ucount lock.

I remember complaining about one version of that patch, but my
complaint wasabout it changing semantics of the saturation logic (and
I think it was also wrong because it still kept the spinlock for
get_ucounts(), so it didn't even take advantage of the atomic
refcount).

Side note: I don't think a refcount_t" is necessarily the right thing
to do, since the ucount reference counter does its own saturation
logic, and the refcount_t version is imho not great.

So it probably just needs to use an atomic_t, and do the saturation
thing manually.

Side note: look at try_get_page(). That one actually does refcounting
with overflow protection better than refcount_t, in my opinion. But I
am obviously biased, since I wrote it ;)

See commits

    88b1a17dfc3e mm: add 'try_get_page()' helper function
    f958d7b528b1 mm: make page ref count overflow check tighter and
more explicit

with that "page->_recount" being just a regular atomic_t.

            Linus