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

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Date:	Thu, 2 Jul 2015 00:34:30 +0300
From:	"Kirill A. Shutemov" <kirill@...temov.name>
To:	David Rientjes <rientjes@...gle.com>
Cc:	Sasha Levin <sasha.levin@...cle.com>, linux-mm@...ck.org,
	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 05/11] mm: debug: dump page into a string rather than
 directly on screen

On Wed, Jul 01, 2015 at 02:25:56PM -0700, David Rientjes wrote:
> 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);

Huh? How would we reach this, if VM_BUG_ON() will trigger BUG()?

> 	}
> 
> 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.

-- 
 Kirill A. Shutemov
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