Re: [PATCH] page-types: decode flags directly from command line

[Wu Fengguang <fengguang.wu@intel.com>]

Hi Alex,

It's easier to illustrate the idea with example and code :)

# Documentation/vm/page-types -d 0x10,anon
0x0000000000001010      ____D_______a_____________________      dirty,anonymous


+static void describe_flags(const char *optarg)
+{
+	uint64_t flags = parse_flag_names(optarg, 0);
+
+	printf("0x%016llx\t%s\t%s\n",
+	       (unsigned long long)flags,
+	       page_flag_name(flags),
+	       page_flag_longname(flags));
+}
 
+		case 'd':
+			opt_no_summary = 1;
+			describe_flags(optarg);
+			break;

Thanks,
Fengguang

On Thu, Nov 05, 2009 at 04:40:08AM +0800, Alex Chiang wrote:
> Hi Fengguang,
> 
> * Wu Fengguang <fengguang.wu@intel.com>:
> > On Wed, Nov 04, 2009 at 06:54:41AM +0800, Alex Chiang wrote:
> > > Why is this useful? For instance, if you're using memory hotplug
> > > and see this in /var/log/messages:
> > > 
> > > 	kernel: removing from LRU failed 3836dd0/1/1e00000000000400
> > > 
> > > It would be nice to decode those page flags without staring at
> > > the source.
> > 
> > In fact it's more than decode - encoding is also possible with the
> > _same_ code! So maybe "-d" and help message will not be all that
> > appropriate.
> 
> I'm sorry, I don't understand this use case, so I'm not sure what
> you're asking me to do.
> 
> You're saying that a use case would be something like:
> 
> 	./page-types --encode referenced,mmap
> 	0x0000000000000004
> 
> ?
> 
> If that's what you're asking for, I guess I'm not sure why that's
> so useful, but then again, I'm a vm n00b so there are probably
> lots of things I don't understand. ;)
> 
> > > Example usage and output:
> > > 
> > > linux-2.6/Documentation/vm$ ./page-types -d 0x1e00000000000400
> > >              flags	page-count       MB  symbolic-flags			long-symbolic-flags
> > > 0x1e00000000000400	         1        0  __________B_______________________buddy
> > >              total	         1        0
> > 
> > The output is a bit redundant - so does the code. Could you simplify
> > them a bit?
> 
> Well, the code is redundant, but add_page() / show_summary() is a
> simple sequence.
> 
> In contrast, I think I'd have to modify walk_addr_ranges() and
> maybe walk_pfn() to do something special when we don't really
> want to do any address space walking, and simply want to
> decode/encode some user input.
> 
> Maybe I don't understand you fully? Could you give me a better
> idea of what you're looking for?
> 
> As for the output, I'm just reusing show_summary(). Maybe we
> don't need the flags, page-count, and MB columns, but again, the
> patch would be more intrusisive because we'd have to teach
> show_summary() about the special case.
> 
> Anyway, I'm happy to make changes closer to what you're looking
> for, but I'd like some more guidance as to what you're expecting.
> 
> Thanks,
> /ac

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