Re: [RFC] [RFT] Shared /dev/zero mmaping feature

[kanoj@google.engr.sgi.com (Kanoj Sarcar)]

> 
> kanoj@google.engr.sgi.com (Kanoj Sarcar) writes:
> 
> > I am not sure why you think the /dev/zero code is a workaround on
> > top of shm. A lot of code and mechanisms are easily sharable between
> > shm and /dev/zero, since they are, as I pointed out, anonymous
> > shared pages. The only differences are when the data structures are
> > torn down, and which processes may attach to the segments.
> 
> Because I think the current shm code should be redone in a way that
> shared anonymous pages live in the swap cache. You could say the shm
> code is a workaround :-)
>

No arguments there :-) But it seems a little ambitious to me to
implement shmfs, dev-zero and rework the swapcache at the same time.
We are probably on the right track, having done /dev/zero, getting 
done with shmfs. Then, if we want to take the risk, we can improve
the core shm/swapcache interactions in 2.3/2.4/2.5.

> > Btw, implementing /dev/zero using shm code mostly is _quite_ easy,
> > that's how the code has been since 2.3.48. Even integrating with
> > shmfs has been pretty easy, as you have seen in the patches I have
> > CCed you on. The harder part is to look towards the future and do
> > what Linus suggested, namely associate each mapping with an inode so
> > in the future the inodecache might possibly be used to manage the
> > shm pages. As you know, I sent out a patch for that yesterday.
> 
> In my opinion this is one of two orthogonal steps. shm fs targets the
> better integration in the file system semantics.
> 
> > Its completely okay by me to take in a dev-zero/shmfs integration
> > patch that is not perfect wrt /dev/zero, as I have indicated to 
> > you and Linus, just so that the shmfs work gets in. I can fix
> > minor problems with the /dev/zero code as they come up.
> > 
> > What sct suggests is quite involved, as he himself mentions. Just
> > implementing /dev/zero is probably not a good reason to undertake
> > it.
> 
> But IMHO reworking shm based on the /dev/zero stuff would be a good
> reason to do the /dev/zero stuff right. That's all I wanted to say in
> my last mail.
> 
> Perhaps I am a little bit too opposed to these changes because I have
> seen too many patches thrown on the shm code during the 2.3 cycle
> which were plain buggy and nobody cared. Most of my kernel work since
> some time is doing quite stupid tests on the shm code.
> 
> BTW: I am just running these tests on your patch and it seems to work
> quite well. (I will let it run over night) If it survives that I will
> also throw some quite complicated /dev/zero tests on it later.

Great! Is there a way to capture tests written by individual developers
and testers and have them be shared, so everyone can use them (when there
are no licensing/copyright issues)? I would definitely like to get your
test programs and throw them at the kernel myself. Lets talk offline if you 
are willing to share your tests.

Kanoj

> 
> Greetings
> 		Christoph
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