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Date:      Tue, 02 Sep 2014 13:31:55 -0500
From:      Alan Cox <alc@rice.edu>
To:        Steven Hartland <killing@multiplay.co.uk>, 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:  <54060D1B.6020700@rice.edu>
In-Reply-To: <B0BE99C514794C24A4D68D4CB12042E1@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>

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On 09/02/2014 12:34, Steven Hartland wrote:
> ----- Original Message ----- From: "Alan Cox" <alc@rice.edu>
>
>> On 09/02/2014 11:01, John Baldwin wrote:
>>> On Saturday, August 30, 2014 1:37:43 pm Peter Wemm wrote:
>>>> On Saturday 30 August 2014 02:03:42 Steven Hartland wrote:
>>>> I'm very disappointed in the attention to detail and errors in the
>>>> commit.  I'm almost at the point where I want to ask for the whole
>>>> thing to be backed out.
>>> I would not be too supportive of that.  This PR has been open for a
>>> long, long time with many users using patches from it in production
>>> loads that were greatly improved by the changes and clamoring on the
>>> lists multiple times to get someone to look at it.  avg@ contributed
>>> quite a bit of time to diagnose this with Karl early on, but other
>>> developers aside from Steven did not.  It also was not hard to
>>> explain to Karl the meaning of 'cache + free' in the bug follow-ups
>>> itself (though I believe avg@ had tried this before and it didn't
>>> sink in that time for some reason).
>>>
>>> I know Steven has since committed a fix, but if there are still
>>> concerns, I think it would be best to not just revert this entirely
>>> but to spend some time  fixing the remaining issues.  Clearly this
>>> issue affects a lot of users and the earlier fixes to pagedaemon
>>> were not sufficient to fix their issues alone.
>>>
>>
>> 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?


> 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.

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.




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