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