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From:   ebiederm@xmission.com (Eric W. Biederman)
To:     "Luck\, Tony" <tony.luck@intel.com>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        Andi Kleen <ak@linux.intel.com>,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        Joe Konno <joe.konno@linux.intel.com>,
        "linux-efi\@vger.kernel.org" <linux-efi@vger.kernel.org>,
        "Linux Kernel Mailing List" <linux-kernel@vger.kernel.org>,
        Jeremy Kerr <jk@ozlabs.org>,
        Matthew Garrett <mjg59@google.com>,
        Peter Jones <pjones@redhat.com>,
        "Andy Lutomirski" <luto@kernel.org>,
        James Bottomley <james.bottomley@hansenpartnership.com>
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Date:   Thu, 22 Feb 2018 11:10:22 -0600
In-Reply-To: <3908561D78D1C84285E8C5FCA982C28F7B37FE28@ORSMSX110.amr.corp.intel.com>
        (Tony Luck's message of "Thu, 22 Feb 2018 05:34:01 +0000")
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Subject: Re: [PATCH] efivarfs: Limit the rate for non-root to read files
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"Luck, Tony" <tony.luck@intel.com> writes:

>>  - add a per-user mutex, and do the usleep inside of it, so that
>> anybody who tries to do a thousand threads will just be serialized by
>> the mutex.
>>
>> Note that the mutex needs to be per-user, because otherwise it will be
>> a DoS for the other users.
>
> I can try that tomorrow (adding the per-user mutex to struct user_struct
> right next to the ratelimit I added.

Another possibility is to cache the files in the page cache.  That will
reduce re-reads of the same data to the maximum extent.

If efi has a chance of changing variables behind our back we might want
something that would have a timeout on how long the data is cached,
and we probably want to make the caching policy write-trough not
write-back.

I just suggest this as it seems like a much more tried and true solution.

Eric