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Date:      Tue, 02 Sep 2014 15:02:48 -0500
From:      Alan Cox <alc@rice.edu>
To:        Steven Hartland <smh@freebsd.org>, John Baldwin <jhb@freebsd.org>, Peter Wemm <peter@wemm.org>
Cc:        svn-src-head@freebsd.org, svn-src-all@freebsd.org, src-committers@freebsd.org, Dmitry Morozovsky <marck@rinet.ru>, "Matthew D. Fuller" <fullermd@over-yonder.net>
Subject:   Re: svn commit: r270759 - in head/sys: cddl/compat/opensolaris/kern cddl/compat/opensolaris/sys cddl/contrib/opensolaris/uts/common/fs/zfs vm
Message-ID:  <54062268.9060603@rice.edu>
In-Reply-To: <774F8EEE96EB483DA8ACF533CF7C23F3@multiplay.co.uk>
References:  <201408281950.s7SJo90I047213@svn.freebsd.org> <E0F163ECBF5E407F99AFDB18FAB05C58@multiplay.co.uk> <39211177.i8nn9sHiCx@overcee.wemm.org> <201409021201.15967.jhb@freebsd.org> <5405FD2C.8000901@rice.edu> <B0BE99C514794C24A4D68D4CB12042E1@multiplay.co.uk> <54060D1B.6020700@rice.edu> <774F8EEE96EB483DA8ACF533CF7C23F3@multiplay.co.uk>

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On 09/02/2014 14:09, Steven Hartland wrote:
> ----- Original Message ----- From: "Alan Cox" <alc@rice.edu>
>> On 09/02/2014 12:34, Steven Hartland wrote:
>>> ----- Original Message ----- From: "Alan Cox" <alc@rice.edu>
>>>> The patch actually makes two completely orthogonal changes at once,
>>>> and
>>>> one of those changes has no connection to the page daemon.  I suspect
>>>> that is why some people have said that their issues were not addressed
>>>> by the page daemon fix.
>>>>
>>>> Prior to this patch, we were limiting the ARC size to 3/4 the kmem
>>>> map/arena/object size on all architectures, including 64-bit,
>>>> uma_small_alloc-enabled architectures where such a limit makes no
>>>> sense.  Consequently, some people were complaining, "Why is 1/4 of my
>>>> memory going unused?"
>>>
>>> This is exactly the problem which lead me into investigating the issue.
>>>
>>
>> Is there any evidence that anything other than eliminating the KVA limit
>> is needed on machines where the page daemon isn't broken?
>
> In "my" direct experience, I would have to say no, I can't speak for
> others.
>
>>> It should be noted that for i386, as requested by Peter, this
>>> limitation
>>> has been re-applied.
>>>
>>
>> Unlike Solaris, we run on a few 32-bit architectures, besides i386, that
>> don't have a direct map or a full 4GB address space for the kernel.  So,
>> for FreeBSD, this needs to be more general than just i386.  I would
>> suggest using '#ifndef UMA_MD_SMALL_ALLOC' as being the closest thing
>> that we have to what you want here.  This check will allow any machine,
>> including 32-bit machines that allocate some kernel memory through a
>> direct map, to opt out of the kmem map/arena/object limit.
>
> I'm not and don't profess to be an expert in this domain as you
> know ;-) So I'm to defer to you guys, Peter would you agree with
> this?
>
>> Finally, the Solaris KVA check is written to avoid the possibility of
>> integer overflow.  However, the FreeBSD version is not.  For example,
>> suppose that I setup an i386 machine with something approaching a
>> 2GB/2GB user/kernel split, 3 * kmem_size() could overflow.
>
> The returns of said functions are uint64_t so I'm not sure where
> the overflow would be when working with 2GB values?
>


Ah, I didn't see the uint64_t.  I only looked at the calculation and
assumed vm_offset_t was being used.  So, yes, you are safe from overflow.


> We seem to be missing some of the following, which doesn't impact
> us directly but may affect understanding of the "current" illumos
> code as its not in the tree:
> https://github.com/illumos/illumos-gate/commit/94dd93aee32d1616436eb51fb7b58094b9a8d3e8
>
>
>    Regards
>    Steve
>
>




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