Date: Wed, 1 Jul 2015 14:25:56 -0700 (PDT)
From: David Rientjes <rientjes@google.com>
To: Sasha Levin <sasha.levin@oracle.com>
cc: linux-mm@kvack.org, akpm@linux-foundation.org,
        linux-kernel@vger.kernel.org, kirill@shutemov.name
Subject: Re: [PATCH 05/11] mm: debug: dump page into a string rather than
 directly on screen
In-Reply-To: <55943DC1.6010209@oracle.com>
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On Wed, 1 Jul 2015, Sasha Levin wrote:

> 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.
> 

I think it's a debate about whether this would be better off handled as

	if (VM_BUG_ON(compound_head(page) != head)) {
		dump_page(page);
		dump_page(head);
	}

and avoid VM_BUG_ON_PAGE() and the new print formats entirely.  We can 
improve upon existing VM_BUG_ON(), and BUG_ON() itself since the VM isn't 
anything special in this regard, to print diagnostic information that may 
be helpful, but I don't feel like adding special VM_BUG_ON_*() macros or 
printing formats makes any of this simpler.
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