Re: [gentoo-dev] Re: [gentoo-kernel] Proper distribution integration of kernel *-sources, patches and configuration.

[Richard Yao <ryao@gentoo.org>]

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On 07/01/2013 09:36 PM, Richard Yao wrote:
> On 07/01/2013 03:23 PM, Greg KH wrote:
>> On Mon, Jul 01, 2013 at 08:45:16PM +0200, Tom Wijsman wrote:
>>>>> Q: What about my stable server? I really don't want to run this
>>>>> stuff!
>>>>>
>>>>> A: These options would depend on !CONFIG_VANILLA or
>>>>> CONFIG_EXPERIMENTAL
>>>>
>>>> What is CONFIG_VANILLA?  I don't see that in the upstream kernel tree
>>>> at all.
>>>>
>>>> CONFIG_EXPERIMENTAL is now gone from upstream, so you are going to
>>>> have a problem with this.
>>>
>>> Earlier I mentioned "2) These feature should depend on a non-vanilla /
>>> experimental option." which is an option we would introduce under the
>>> Gentoo distribution menu section.
>>
>> Distro-specific config options, great :(
>>
>>>>>    which would be disabled by default, therefore if you keep this
>>>>> option the way it is on your stable server; it won't affect you.
>>>>
>>>> Not always true.  Look at aufs as an example.  It patches the core
>>>> kernel code in ways that are _not_ accepted upstream yet.  Now you all
>>>> are running that modified code, even if you don't want aufs.
>>>
>>> Earlier I mentioned "3) The patch should not affect the build by
>>> default."; if it does, we have to adjust it to not do that, this is
>>> something that can be easily scripted. It's just a matter of embedding
>>> each + block in the diff with a config check and updating the counts.
>>
>> Look at aufs as a specific example of why you can't do that, otherwise,
>> don't you think that the aufs developer(s) wouldn't have done so?
> 
> I am accquainted with the developer of a stackable filesystem developer.

I should probably proofread multiple times before I send emails. Anyway,
that should have been:

> I am acquainted with the developer of a stackable filesystem.

> According to what he has told me in person offline, the developers on
> the LKML cannot decide on how a stackable filesystem should be
> implemented. I was told three different variations on the design that
> some people liked and others didn't, which ultimately kept the upstream
> kernel from adopting anything. I specifically recall two variations,
> which were doing it as part of the VFS and doing it as part of ext4. If
> you want to criticize stackable filesystems, would you lay out a
> groundwork for getting one implemented upon which people will agree?
> 
>> The goal of "don't touch any other kernel code" is a very good one, but
>> not always true for these huge out-of-tree kernel patches.  Usually that
>> is the main reason why these patches aren't merged upstream, because
>> those changes are not acceptable.
> 
> I was under the impression that there were several reasons for patches
> not being merged upstream:
> 
> 1. Lack of signed-off
> 2. Code drop that no one will maintain
> 3. Subsystem maintainers saying no simply because they do not like
> <insert non-technical reason here>.
> 4. Risk of patent trolls
> 5. Actual technical reasons
> 
>> So be very careful here, you are messing with things that are rejected
>> by upstream.
>>
>> greg k-h
>>
> 
> Only some of the patches were rejected. Others were never submitted. The
> PaX/GrSecurity developers prefer their code to stay out-of-tree. As one
> of the people hacking on ZFSOnLinux, I prefer that the code be
> out-of-tree. That is because fixes for other filesystems are either held
> back by a lack of system kernel updates or held hostage by regressions
> in newer kernels on certain hardware.
> 
> With that said, being in Linus' tree does not make code fall under some
> golden standard for quality. There are many significant issues in code
> committed to Linus' the kernel, some of which have been problems for
> years. Just to name a few:
> 
> 1. Doing `rm -r /dir` on a directory tree containing millions of inodes
> (e.g. ccache) on an ext4 filesystem mounted with discard with the CFQ IO
> elevator will cause a system to hang for hours on pre-SATA 3.1 hardware.
> This is because TRIM is a non-queued command and is being interleaved
> with writes for "fairness". Incidentally, using noop turns a multiple
> hour hang into a laggy experience of a few minutes.
> 
> 2. aio_sync() is unimplemented, which means that there is no sane way
> for userland software like QEMU and TGT to be both fast and guarantee
> data integrity. A single crash and your guest is corrupted. It would
> have been better had AIO never been implemented.
> 
> 3. dm-crypt will reorder write requests across flushes. That is because
> upon seeing a write, it sends it to a work queue to be processed
> asynchronously and upon seeing a flush, it immediately processes it. A
> single kernel panic or sudden power loss can damage filesystems stored
> on it.
> 
> 4. Under low memory conditions with hundreds of concurrent threads (e.g.
> package builds), every thread will enter direct reclaim and there will
> be a remarkable drop in system throughput, assuming that the system does
> not lockup. There is a fairly substantial amount of time wasted after
> one thread finishes direct reclaim in other threads because they will
> still be performing direct reclaim afterward.
> 
> 5. The Linux 3.7 nouveau rewrite broke kexec support. The graphics
> hardware will not reinitialize properly.
> 
> 6. A throttle mechanism introduced for memory cgroups can cause the
> system to deadlock whenever it is holding a lock needed for swap and
> enters direct reclaim with a significant number of dirty pages.
> 
> 7. Code has been accepted on multiple occasions that does not compile
> and the build failures persist for weeks if not months after Linus' tag.
> I sent a patch to fix one failure. It was rejected because I had fixed
> code to compile with -Werror, people thought that -Werror should be
> removed (and therefore was no reason to fix the warnings) and we went 2
> months until someone wrote a patch that people liked to fix it. For a
> current example of accepted code failing to build, look here:
> 
> https://bugzilla.kernel.org/show_bug.cgi?id=38052
> 
> Note that I have not checked Linus' tree to see if that bug is still
> current, but the bug itself appears to be open as of this writing.
> 
> There are plenty more technical issues, but these are just my pet
> peeves. If you want more examples, you could look at the patches people
> send you each day and ask yourself how many are things that could have
> been caught had people been more careful during review. For instance,
> look at the barrier patches that were done around Linux 2.6.30. What
> prevented those from being caught by review years earlier?
> 
> Being outside Linus' tree is not synonymous with being bad and being bad
> is not synonymous with being rejected. It is perfectly reasonable to
> think that there are examples of good code outside Linus' tree.
> Furthermore, should the kernel kernel choose to engage that out-of-tree

That should have been:

> Furthermore, should the kernel team choose to engage that out-of-tree

> code, my expectation is that its quality will improve as they do testing
> and write patches.
> 



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