linux-kernel - Re: [PATCH 05/11] mm: debug: dump page into a string rather than directly on screen

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Date:	Wed, 01 Jul 2015 15:21:37 -0400
From:	Sasha Levin <sasha.levin@...cle.com>
To:	David Rientjes <rientjes@...gle.com>
CC:	linux-mm@...ck.org, akpm@...ux-foundation.org,
	linux-kernel@...r.kernel.org, kirill@...temov.name
Subject: Re: [PATCH 05/11] mm: debug: dump page into a string rather than
 directly on screen

On 06/30/2015 07:35 PM, David Rientjes wrote:
> I don't know how others feel, but this looks strange to me and seems like 
> it's only a result of how we must now dump page information 
> (dump_page(page) is no longer available, we must do pr_alert("%pZp", 
> page)).
> 
> Since we're relying on print formats, this would arguably be better as
> 
> 	pr_alert("Not movable balloon page:\n");
> 	pr_alert("%pZp", page);
> 
> to avoid introducing newlines into potentially lengthy messages that need 
> a specified loglevel like you've done above.
> 
> But that's not much different than the existing dump_page() 
> implementation.
> 
> So for this to be worth it, it seems like we'd need a compelling usecase 
> for something like pr_alert("%pZp %pZv", page, vma) and I'm not sure we're 
> ever actually going to see that.  I would argue that
> 
> 	dump_page(page);
> 	dump_vma(vma);
> 
> would be simpler in such circumstances.

I think we can find usecases where we want to dump more information than what's
contained in just one page/vma/mm struct. Things like the following from mm/gup.c:

	VM_BUG_ON_PAGE(compound_head(page) != head, page);

Where seeing 'head' would be interesting as well.

Or for VMAs, from include/linux/rmap.h:

	VM_BUG_ON_VMA(vma->anon_vma != next->anon_vma, vma);

Would it be interesting to see both vma, and next? Probably.

Or opportunities to add information from other variables, such as in:

	VM_BUG_ON_PAGE(stable_node->kpfn != page_to_pfn(oldpage), oldpage);

Is stable_node->kpfn interesting? Might be.


We *could* go ahead and open code all of that, but that's not happening, It's not
intuitive and people just slap VM_BUG_ON()s and hope they can figure it out when
those VM_BUG_ON()s happen.

Are there any pieces of code that open code what you suggested?


Thanks,
Sasha
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