\nThen, when we tried to execute a task from the shell, things exploded.
\nAfter about an hour, I tracked down the problem to fork’s page-directory-and-table copying function… odd though, since that hadn’t changed since before the VM rewrite, and it was working fine before.
\nAfter poking at it some more, I determined that, for some unknown reason, one of the page tables in the newly-forked task contained garbage data. Garbage data that happened to look a lot like a kernel stack. Hmmm…
\nA bunch of breakpoints and plenty of
x%cr3 4096<\/code> and psym \"pcb->esp0\"<\/code> later, I discovered that, somehow, the same physical page was being mapped as both a kernel stack page and as a page table page. Very odd.
\nPoked at it for another couple hours until I managed to isolate the problem to one function… a function that destroys the current mappings in the page directory\/tables to prepare a task for exec. Turns out it was a mistake with freeing pages… for some reason, the code was freeing a page *after* the desired page (PTR_ADD(page_loc, PAGE_SIZE)<\/code>). Oops. Fixing that fixed the immediate bug but tripped an assertion.
\nThat one turned out to be simpler… the wrong virtual address was being passed into the freeing function (*entry<\/code> instead of entry<\/code>). Whee.<\/p>\nSo yeah, it appears that we are back in the same state that we were yesterday as far as runnability and such goes, except we have a shiny new VM that appears to be much happier (and much cleaner) than before.<\/p>\n
I should probably go sleep, as it is 7:30… but it might make more sense to just stay up until class and then sleep afterward. Meh.<\/p>\n
All we have left to do is finish vanish and write wait and we will be kernel code-complete (at least, theoretically… we’ll probably end up doing more rewrites and such). Awesomeness.<\/p>\n
There has been much ITG lately. I need to get back into 11s… maybe I’ll manage to pass a 12 before I leave CMU. Wheee.
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<\/p>\n","protected":false},"excerpt":{"rendered":"
Don’t you love that sense of satisfaction you get after you fix a rather annoying bug? 8 finished the VM rewrite for our kernel (and it looks *much* nicer now, and the PM can also magically keep track of free … Continue reading