lkpi_vmf_insert_pfn_prot_locked(): detect uninitialized pages
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Authored by kib on Tue, Sep 8, 12:18 PM.

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alc
markj

Summary

This would not detect an unitialized page at the physical address zero,
but otherwise should be reliable.

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rG FreeBSD src repository

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kib created this revision.Tue, Sep 8, 12:18 PM

Herald added subscribers: linuxkpi, emaste, imp. · View Herald TranscriptTue, Sep 8, 12:18 PM

kib requested review of this revision.Tue, Sep 8, 12:18 PM

I understand that the immediate objection would be that memset's on the vm_page_array[] are slow and somewhat against the lazy initialization of vm_pages. But this is plain memset instead of the CPU-intensive calculations of proper vm_pages.

This should work against the class of drivers bugs where vma is created against a region that is not registered with the fictitious managed range. I was bitten by drm 6.12 on my new (but quite old) KabyLake notebook.

kib added reviewers: alc, markj.Tue, Sep 8, 12:21 PM

bz added a subscriber: bz.Tue, Sep 8, 12:49 PM

What about powerpc?

I don't really understand how this fixes the problem: even if a fictitious range was registered, PHYS_TO_VM_PAGE will try to find the page from the vm_page_array first. What exactly is the driver bug?

Can we make the zeroing conditional on INVARIANTS? Alternately, maybe @cperciva would be willing to benchmark this change in EC2, that was where a lot of the work described here was tested: https://wiki.freebsd.org/BootTime