Saturday, January 23, 2010

OpenSolaris sources are beautiful

Trying to find the roots of the "hsfs_putpage: dirty HSFS page" error, I looked in the OpenSolaris source.

High Sierra is a pretty old and stable stuff, so it is possible that the code is similar to OpenSolaris.
I looked in debugger, and the function calls hierarchy looks pretty similar.

Now in the OpenSolaris source code there is a nice comment:

* Normally pvn_getdirty() should return 0, which
* impies that it has done the job for us.
* The shouldn't-happen scenario is when it returns 1.
* This means that the page has been modified and
* needs to be put back.
* Since we can't write on a CD, we fake a failed
* I/O and force pvn_write_done() to destroy the page.
if (pvn_getdirty(pp, flags) == 1) {
                           "hsfs_putpage: dirty HSFS page");

The bright side: I don't know any other open source project which would be so nicely documented. The description confirms the suspect I had: it's the problem with MMU emulation.

The dark side:  it's not just the problem with hsfs. Other file systems will have this bug too, and there it must be even more dramatic: they must be constantly writing cache data back to disk.

The 100% mmu & mxcc emulation in qemu would make the memory access very slow. I still hope we can avoid this, but don't know how.

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