Re: [Xen-devel] Merging xen/dom0/backend/blktap2 ..

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

On Sat, Mar 19, 2011 at 04:20:45PM -0700, Daniel Stodden wrote:
> On Tue, 2011-03-15 at 23:02 -0400, Konrad Rzeszutek Wilk wrote: 
> > > > Can you make a different branch (or just send me the git commit)
> > > > for the one that touches mm/memory.c?
> > > 
> > > That's the initial one: 7ccd87f. You could strip that but it's not going
> > > to build as a module before that ref is gone. It might go at some point.
> > 
> > For upstream we need that as a separate git commit. Otherwise we might
> > not get MM maintainers Ack-ing the git commit b/c they would have to review
> > the blktap driver too. Please do split it out in two commits.
> > 
> 
> This piece is not ready for upstream, but you'd be right.
> 
> Proposing a zap_page_range export to mm maintainers will produce a
> simple "you're doing it wrong", let's not split commits just for that.
> 
> My apologies, that detail dates back to the dawn of xen-blktap1.
> Userspace is used to mapping a single large VMA, and the driver flips
> ptes implicitly as I/O reqs leave and return again. That's machine-wise
> not much of a difference, but for normal memory there's mmap/munmap
> already, so module code is usually not encouraged to invent funny new VM
> area models. Hence, zap_page_range remains private.
> 
> Can it just resort to mmap/munmap? Probably, even implicitly from ioctl
> context I guess.

The gntdev solved this by clearing the PTE on unmap (using the gnttab API and
the M2P/P2M override which clears the PTE).

Would that work?
> 
> If it hopelessly got in your way, one could back it out, the loss is
> TAP=m.
> 
> > > 
> > > > > 
> > > > > Does this sound acceptable?
> > > > 
> > > > I looked briefly at blktap/next-2.6.38 and it looks like an ongoing
> > > > merge tree. Can you make a tree that is based off 2.6.38 (or some
> > > > branch of my 'stable/' ones). Basically trying to have something that is
> > > > easy to merge and is self-containted within one branch.
> > > 
> > > It is rooted on 2.6.32 and incrementally moves on up to currently
> > > v2.6.38, no stray diffs except the forward porting where needed. There's
> > > at least one conflict resolution in Kconfig around 36 or 37, iirc. Any
> > > later kernel will look more or less the same.
> > > 
> > > $ git list xenbits/blktap/next-2.6.38 ^v2.6.38
> > > eb8040c Merge commit 'v2.6.38' into blktap/next-2.6.38
> > > bf926e6 blktap: Forward port to 2.6.37
> > > 9f65e90 Merge commit 'v2.6.37' into blktap/next-2.6.37
> > > 7ef4e35 blktap: Forward port to 2.6.36
> > > a55f064 Merge commit 'v2.6.36' into blktap/next-2.6.36
> > > 994a546 blktap: Add discard queue limits.
> > > ea2d7e7 Merge blktap into blktap/next-2.6.33
> > > e37d737 blktap: Add BLKTAP_OP_TRIM command option.
> > > 42276df blktap: Add BLKTAP_OP_FLUSH command option.
> > > 5d10876 blktap: Add physical sector device info.
> > > 8169b25 blktap: Drop the ring message timestamp.
> > > 34574a8 blktap: Support non-R/W requests
> > > a765257 blktap: Fix reference to freed struct request.
> > > 7ccd87f blktap: Blktap userspace devices.
> > > 
> > > You could rebase it on v2.6.38 if you want to reproduce the merging and
> > > mangle commits, although I'm not sure why. For the foreseeable future,
> > > I'd like to keep 2.6.32 and anything later going, so that repo will get
> > > incoming stuff at 2.6.32 and, where possible, I'd expect the greater
> > 
> > Ah, ambitious. I tried that with xen-pcifront and had to resort
> > to making trees for different versions. But this could work for you.
> 
> But it should work fine here. There's only a single source tree, which
> is linux-2.6.git, and only a single component, which aims to yield the
> same extensions where possible. Doesn't get much easier than that.
> 
> I'd agree that components interfacing with 4 moving xen subsystems, as
> in blkback etc. is a bit of a different matter.
> 
> Still, killing history altogether can't be the answer.

Seems to be the answer nowadays. David Miller asked Ian to squash the full
history in one commit, and provide a "historic" git tree where all of the
git commits reside.

> 
> > > kernels to typically just fast-forward from where they are now.
> > 
> > The issue is git bisection. The dates on those commits is quite early so
> > I wonder if somebody did a git bisect whether they would hit those and
> > end up with compilation issues. Perhaps the first patch should
> > just introduce the driver, but not touch the Kconfig at all.
> > 
> > And then the last one actually enables the Kconfig. And as you
> > add updates, you move the patch with Kconfig to be the last one?
> > 
> > > 
> > > If you want topic branches: I'm not planning to do those. Mainly because
> > > it's a bad idea. If that entire thing would be about to go upstream with
> > > limited time to land that might a different thing, but it's clearly not
> > > there in its present state.
> > 
> > What is a "bad" about it? What do you want to do when the driver is ready
> > for upstream? You would have to do this topic branch at some point, 
> > wouldn't you?
> > What is a topic branch to you? When I think topic branch I think:
> > devel/xen-pciback-0.5 for example.
> 
> When (iff) that driver gets ready for upstream I guess it'd have to be
> folded down and optionally split, pretty much exactly as you describe.
> 
> This is my third attempt to write a proper response to your concerns,
> because so far I'm not super-experienced with git either.
> 
> I was going to disagree because keeping separate queues is a bit of a
> workflow killer.
> 
> Then I was going to agree because commit atomicity for proper bisection
> is a actually pretty good point, imho.
> 
> The thing I hate about rebasing/cherry-picking is not even the extra
> work, it's that the dupes mean one loses any kind of traction wrt what's
> in a particular derived tree and not. So in order to be able to compare
> two such branches one has to diff sources, git-blame, and understanding
> a whole lot too much about the content.

Or:
git shortlog devel/xen-pciback-0.3..devel/xen-pciback-0.5  --grep="pciback"

Which gives you some good idea of what was added... that is if one
does not mess with the git commits and amend them (which I sometimes too
since they are still "devel").

> 
> The commit atomicity issue could be solved by making sure stay in a
> branch, and only get merged, not pulled through by fast-forward. That
> git-merge --no-ff option looks like it was meant to ensure that.
> 
> There's another way to make stuff atomic: Amending merges with the
> following port. A git-commit --amend can do that. Took me ages to find
> that out, because for reasons unclear, git-rebase -i alone (as 'fixup'
> changes) can't.
> 
> I tried whether that's better. Can't recommend it. Git appears to have a
> really hard time turning such ports/merges back into diffs, and having
> those diffs is quite useful. It's not impossible, but format-patch alone
> can't, which gets a bit counterintuitive.
> 
> > The reason I am asking about this is b/c of my inept git skills. Doing
> > "git pull daniel/blktap" and just having it fetch patches that are relevant
> > to blktap makes it soooo much easier to get an idea what is happening.
> 
> Well, it's not like it's going to pull garbage. These are all blktap
> exclusively, rooted at v2.6.38. Just not ported individually, but in
> piles.

OK. The desciption were a bit generic: "porting to 2.6.37" .. doesn't really
tell what you had to do, or what not. I did not look in the description, so
it might have had the details.

> 
> Let's assume one would go through the pain to port those one after one,
> by cherry-picking and careful rebuild. With 2.6.39 one had to start over
> again, porting every commit once again, and making sure it compiles to
> stay consistent, again for bisection purposes. And so on, every couple
> months at least.

That can be automated quite easily. The .git/hooks can do the make process
automatic before checking it, and failing if it is not working.

So once you set that up, it is as simple as 'git rebase -i v2.6.39'...
but I am not sure if that is needed.

The reason you might need to rebase it is if the newer tree has thrown some
API out or introduced a new one that the blktap can use. But if that is
not the case, there should be no trouble on a 2.6.40 tree to do:

git merge <devel-tree-based-on-2.6.36> .. and fix up the little merge
issues (if any).

> 
> That continues, until this thing gets either dropped or upstreamed, at
> which time an O(n*m) disaster gets folded down and discarded, one way or
> the other. That not very efficient.

The whole upstream effort is not efficient at all :-) It sometimes
is akin to trying to convience a tax auditor that they are wrong.

> 
> The reality is it looks like git can't deal with it any better that what
> I got there.
> 
> So I'd rather ask you open a konrad/blktap-2.6.38, tracking that branch,
> and carefully merge that in, in a way not risking bisection.

OK. Bisection is important - especially during those rc-X cycles where
stuff stops working and is a tangle to figure out what went wrong.

And it sounds to me that you want to defeer the idea of upstreaming
for some time, and when you are comfortable with blktap2 being in great
shape - then start the process.

When it comes to upstreaming process the fashion looks to post one
nice patch that includes the driver. Infrastructure patches should be
done as seperate patches. So at that point it probably won't matter
at all whether you have little bits of patches, or just one big one.
The maintainer (Jens Axboe) will probably just pick the big one.

> 
> If that's okay, next question would probably be if one shouldn't try do
> the same with blkback >:)

For authorship I prefer to keep branches. So say I post the blbkback
driver "as is" for review. People come back with reviews, or what not - I create
a new branch with a new patch with the review feedback. That way when
I am finally done, I've this "historic" branch that has all the names of
people who contributed to it.

There is a twist to all of this. Linus himself prefers to ingest whole
git branches. Other maintainers, like just one single patch.

Let me ping Jens Axboe and find out what he prefers. That should
give us a good idea of where to continue.

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