On Wed, 1 Jul 2015, Sasha Levin wrote:
> Since we'd BUG at VM_BUG_ON(), this would be something closer to:
>
> if (unlikely(compound_head(page) != head)) {
> dump_page(page);
> dump_page(head);
> VM_BUG_ON(1);
> }
>
I was thinking closer to
if (VM_WARN_ON(compound_head(page) != head)) {
...
BUG();
}
so we prefix all output with the typical warning diagnostics, emit
whatever page, vma, etc output we want, and then finally die. The final
BUG() here would have to be replaced by something that suppresses the
repeated output.
If it's really just a warning, then no BUG() needed.
> But my point here was that while one *could* do it that way, no one does because
> it's not intuitive. We both agree that in the example above it would be useful to
> see both 'page' and 'head', and yet the code that was written didn't dump any of
> them. Why? No one wants to write debug code unless it's easy and short.
>
pr_alert("%pZp %pZv", page, vma) isn't shorter than dump_page(page);
dump_vma(vma), but it would be a line shorter. I'm not sure that the
former is easier, though, and it prevents us from ever expanding dump_*()
functions for conditional output.
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